I had a spines/DA untill I rolled one in the RV event. The endurance dude! It just can't be balanced well enogh to sit back and have scrapper PvP fun. On this basis I would say MA.
*ahem*.
If you wish to go 1v1 PVP, /dark is not your friend. Sure, it can hold its own against most other scrappers (yes, I HAVE fought against /regens without losing), anything /psi is going to lose pretty much automatically, but blasters have the burst damage to destroy you fairly quickly. Having said that, it's a very utlitarian set, and it has served me well in zone PVP. (To the tune of beating down multiple stalkers and doms, granite brutes, and a few masterminds)
MA/DA is a choice I myself have just started, and am looking forward to. With a combination of boxing, OG, and other moves from MA, it's possible to hit mag 8 stun, which is enough to detoggle anything but a /regen scrapper (although some tanks might be an issue, not checked).
Spines/DA is the single most awesome thing in this game. The damage! build up+Quills+death shroud+spine burst (all slotted for damage where appropriate) reaches a brawl index of 13. In one single tic. Given the time taken to set spine burst up, it's more like 16-19 brawls of damage to up to 8 opponents. That's a BI of 152. One hundred and fifty two. In about 10 seconds. Or, an DPS of 15.2, when in optimal conditions (IE, whenever build up is charged and you are surrounded by 8 mobs). Spines/DA is the KING of AoE damage. However, the endurance is ludicrous, and unless you have a pet kin or a mountain of blues I advise against it.
As usual, I will throw in the word for DM/DA. It has the highest DPS in the game (tested on the American forums with hasten I believe), and serious ultility. With the stacked debuffs, I can gain the equivalent of double slotted elude, from a measly cloak of darkness.
But as for powers to pick: Not really knowledgable about spines or MA, so I'll stick to the DA (and DM if you're interested 
).
Dark Embrace: Your main defence. Gives some mediocre S/L resist, as well as some of your negative energy resist. Fairly low cost, so I'll say slot 3 resist/1 end redux
Death Shroud: This is fun early game (arrest 5 minions in 10 seconds) and devastating late game (arrest 8 bosses in under a minute). The endurance is a pain though. I don't plan on getting this until I'm raiding Hamidon, and can get some Nucleous exposures to raise the Acc and Damage of the move. This is more damage and almost half the end cost of quills, so if you want an aura for damage and not the -recharge, pick this. If I had it, I would go 1acc/2dam/3end redux.
Murky Cloud: Your basic element/energy/end drain resist. This is the thing that makes /regens scream in envy as a sapper demolishes them, and you're left to solo the malta. You'll survive it too. 
It also gives your some more neg resist. 3 resist/1 end redux.
Obsidian Shield: If Murky Cloud made regens scream, this makes everyone else just break down and cry. It's your standard sleep/disorient/hold resist (missing knockbac) but also resists fear, and psi. Unslotted it has 37.5% psi resist, which means tanking carnies and anything else like that is no hassle if your keep an eye on your HP. It's just so awesome to fight anything that uses Psychic attacks and see "7 damage" tick up from mind probe
Standard shield slotting, 3 resist/1end redux.
Dark Regeneration: Start drooling. A PBAoE that has NO accuracy reduction to it, and heals 30% health per mob. 3 mobs= pretty much full health. The damage is mediocre, and never worth slotting. The heal is uber, and doesn't need slotting (IMO, YMMV, ETC). It's on a 30 second recharge timer. I like to spam it while surrounded by 17 +2s, so I go 3acc/3recharge. But you could always go 2acc/2heal/2recharge. For 1v1 PVP, I would go 3 heal/2acc/1recharge.
Cloak of Darkness: Your immob resist, fairly high mag. And it's got the combat jumping defence bonus. And it's stealth. It boosts your perception. It's cheap. For a PVE build, it's skippable because while being good, it's not great. For PVP it's free tactics. Take it, love it.
I slot it with just 1 end redux, nothing else worth slotting.
Cloak of Fear: Ohmigodwtfbbqpwnz. This thing just stops minions moving. It's beautiful.
It's also competing for "heaviest end usage toggle in existance", has a 50% tohit, had it's acc debuff nerfed (but it's still 10%, fairly decent), and the fear isn't all too long. For a Dark/Dark, when you have ToF with it it's brilliant. It can even (on it's own) interrupt an AS by fearing the stalker (which makes them visible).
It's a good power, but not brilliant. I can't afford it in my build, and with Spines/ I doubt you will be able to. If you do, 3acc/3endurance. It's too end heavy and innacurate to slot otherwise viably.
Oppresive Gloom: Holy [censored]. This thing is just superb. Stuns minions, for almost no end cost and a tiny health cost. I respecced out of it and missed it, this thing could easily stun a squishy all by itself. Take it. And I would slot 3acc/3disorient, endurance reduction is minimal.
Soul Transfer: Some hate this, some love it. Hell, I used to hate it and now love it, with it's brute defeating properties in RV. It basically gives you 30% health and endurance for every enemy it hits, which it then lands with a mag 37 stun, and makes you immortal for a (very) short while. I am still going to spec out of it though, don't have the room. But I will miss it.
Slot it with one recharge, and as many disorients and extra recharges you can. I think of this as an "I win" button against whatever took me out, not a resurrect.
Hope that helps.
EDIT: forgot 2 things. One: You need leaping pool. Extra immob protect always rules, and you need Acro for KB protection and to stop blasters being able to hold you. I consider this next to stamina in usefulness.
Two: I present The DM/DA bible. Has some advice that differs from me, if so take his (he's been doing it longer)
dark destroyer is only lvl and not my main since 3 weeks ago but i really respect u wnting to be different and da will serve u well cloak of darkness i hav hours of fun with in sirens call its just another stealth but it looks cooler but i get to give those stalkers a taste of there own medicine
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Death Shroud: This is fun early game (arrest 5 minions in 10 seconds) and devastating late game (arrest 8 bosses in under a minute). The endurance is a pain though. I don't plan on getting this until I'm raiding Hamidon, and can get some Nucleous exposures to raise the Acc and Damage of the move. This is more damage and almost half the end cost of quills, so if you want an aura for damage and not the -recharge, pick this. If I had it, I would go 1acc/2dam/3end redux.
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FYI the -recharge on Quills is in the region of 6% over time. Not worth getting for the mez effect IMO, get Death Shroud first, although the 2 of them together would be lovely. For -recharge it is Throw Spines that makes the most difference ( -20% recharge ).
@Unthing ... Mostly on Union.
Not sure about what it does for PvP or anything like that, i chose my powersets for the character concept (and i'm not that far into the character anyway) but I have a DM/SR scrapper and I find very little I can cope with on my own (as I said, i'm not far in so the worst is still to come )
The damage is nice and Siphon Life and Dark Consumption allow the gain of health and Endurance mid-scuffle. Which has got me through some tough fights with multiple yellows (with the help of a few insps)
Plus DM looks cool...
Here I am.. rolling another alt. After a Ice/Nrg Tanker and a Dark/Rad Defender, this time I'm rolling a Scrapper. As I wanted something different from the typical (even if uber and fun) Spines/Regen, I was thinking of a MA/DA or Spines/DA. I do want to PvP later on (L30+) so I was wondering if DA can do well. I've allways been a fan of Dark sets (even if some people say it suks for PvP) and according to the description of the powers I really think they should do nicely. Could anyone suggest a build or help me decide between Spines and MA. Thanks in advance.
@False Fiction - Virtue / Defiant
Current projects - [Glaciologist - Ill/Cold Troller] [Cloudshaper - Storm/Dark Def] [Harald Wartooth - Elec/Psi Domi]