What's your primary/secondary? Also, PvE exclusively or some PvP as well?
The snipes, in general, suck. They take WAY too long (8 seconds, seriously) for the damage they do. You're better off with the single target blast. Dark Blast fires off VERY quickly for good damage, Shadow Meld is an excellent power for any non-soft capped character (particularly /regens) and Soul Storm just looks neat.
Waterspout can make a handy "Oh crap!" power and adds a nice bit of chaos to the mix. Hibernate is the ultimate "NO U!" power to any and all incoming damage, and Spirit Shark Jaws is probably one of my favorite holds in the game. Get a Kronos Titan held in the LRSF, then use that on it for king sized OM NOM NOM!
"the reason there are so many sarcastic pvpers is we already had a better version of pvp taken away from us to appease bad players. Back then we chuckled at how bad players came here and whined. If we knew that was the actual voice devs would listen to instead of informed, educated players we probably would have been bigger dicks back then." -ConFlict
I'll give ya my 2 cents on it since i have used both. I personally prefer mako for a stalker over GW, but to each his own.
Ghost widow:
Yes, the snipe sucks: looong cast time, loooong rech and meh damage. It's like tier 2 blast damage. Altho the ghost widow snipe -does- look cool. Soul storm is the ultimate looking hold in the game, but it has a long animation. No one will have a problem noticing the mob is held tho heh.
Shadowmeld is nice if your a resistance based toon or dont have softcapped defenses, otherwise is skippable. Dark blast is a nice ranged attack and can be easily put into an attack chain (however redraw for DBs, claw etc can suck).
Mako:
Waterspout is crazy, it's like unleashing a madman into a battle. Its basically a panic button because of its insane knockback, but can do decent dmg if the targets are immobilized w/ a -knockback component.
Hibernate is an ok power, I'm not one for phase powers but if you have no heal or have end problems i can see a use for this. I've had it before and kept forgetting to use it heh, with low hps a stalker really needs to pay attention to his health and gauge the difference between when it's appropriate to use. At lvl 50 a melee mob can knock off half your health in one shot, but I'd never consider using hibernate at 50% health for example. Decent for pvp i hear.
Spirit shark is -great- for a few reasons: knockback and fast recharge. If you are having problems with a boss you can keep it knocked on its butt permanently with this ability. Also, in pvp this powe does insane damage (like 180 per shot, plus it crits from hide).
Spirit shark jaws are awesome. It's cast time is very fast and it will bring down flying opponents (nothing quite like a 30 foot transparent shark leaping out of the ground and pulling a freakshow juicer out of the sky for a nice buttwhooping).
Both sets have pets, I haven't used either one but I'm sure they have their uses (if anything, the purple pet IOs aren't terribly expensive and 4 of them get you 16% regen, 45 hps and a 4% damage buff).
Again, I prefer mako as I think it is a little more effective. It's the more pvp orientated of the two as well, something to consider if you participate in that.
Thanks for the help guys I forgot I'm EM/Nin and I have 2 heals (took Medicine) and I have Retsu as my Oh S*** power I just can't decide lol but Leviathian does sound interesting I loved Ice Patch on my Dom just don't think I was Hibernate but I guess I can do the Water Spout, Spirit Shark Jaws and Smoke flash from Nin or take Spirit Shark. I'm really thinking about taking both in 2 builds but I don't even have the cash for this toons enhacements (I was in Sg mode before the black market and never played with this toon since.) But I guess over time I can slot up both builds or maybe do test
I forgot to mention how is Shadow Meld? I was all set to take Leviathian and then remembered that shadow meld sounds really good and I hear with enhancements I can get the recharge down to 30 seconds. Enough for even more +defense though it doesn't last too long
Shadow Meld is really good for a /Nin if you arent soft-capped already. If you are on a team without a good Brute you can use it and AS something and usually only 1 or 2 (if any) attacks will hit, and a lot of the mobs will have fear from demoralize. If your team is any good, by the time it wears off enough mobs should be dead and/or aggroed to other people for you to easily survive. And then its up by the next fight easily 
I've narrowed it down to 2 Levianthan and Soul. I'm trying not to respec but I guess I could do 2 seperate builds and take both. My thinking for Soul is Moonbeam nice damage if I put some Interrrupts in there I can use it in battle. Shadow Meld and Soul Storm are pretty obvious.
As for Leviathian I was thinking Water Spout (it sounds like Ice Patch but with Disorient and Fear) which sounds nice. After running into a battle and taking some guys out. Spirit Shark Jaws should be good and Hibnerate should be okay but I don't think I need it too much just as an oh crap move. What do you guys think? Is there a Patron most Stalkers take?