"It depends"
Fire is more damaging PvE, since you get fiery embrace and a lot of AoE damage.
Thorn is slightly better PvP
Primary can make a lot of difference - Plant/Fire is incredible fun and some of the easier powers fit really well... I've never played /thorn but flith did and he was surprisingly effective in PvP ... at least, everybody he fought against found it surprising 
Synaesthetix:if your mum wasn't already dead I would go kill her for bringing
you into the world
for primary i hesit between fire and grav but and yes it would be for a try in PvP map ^^'
I went for a Fire/Fire offensive build....and i've never had so much fun in ages on a toon, Domination+Firey Embrace=Crazy Damage.
Just got my crazy lil Fire Minkies and i now solo faster than i did on my Corr at this lvl.
The problem i see with the Grav primary is that it always seemed to be the solo'ist/Damage dealers choice because of the high brawl index of Propel (sorry if thats wrong), but now with an Assault secondary i feel thats its now a redundent power on a set that seemed lacking a little...peoples opinions will differ from mine, but this just thoughts.
Try a fire/fire and go cleanse the world in flames!
I can understand propel being a pointless power for a dominator. You have enough attacks in your secondary and the animation is so long it's very bad for building domination. Still looks damn cool though.
I miss him in the weeping of the rain;
I want him at the shrinking of the tide;
The old snows melt from every mountain-side,
And last year's leaves are smoke in every lane;
But last year's bitter loving must remain
~Edna St. Vincent Millay
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for primary i hesit between fire and grav but and yes it would be for a try in PvP map ^^'
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If you want to pvp, dont take fire as primary.. I think it is too lacking in the control department to be honest.
Grav would be better.
I've only played Thorny Assault to 50, the other sets to the teens.
Pros:
It's a high damage single target set, and has a pretty good cone attack (Thorn Burst is lousy though). You get Skewer and Impale very early on, and the last two attacks are killer.
Soft Controls - Impale is a very good debuff & control attack. Thorntrops is the same as Caltrops. Every attack has some -Def element, some more than others.
Poison damage - not easily resisted and with some attacks poison can be about 50% of the damage.
Accuracy - Thorns is the only set which has Aim which means it is the most accurate of all Dominator assault sets, this has good synergy with the -Def debuffs you get in the attacks. Aim buffs damage some as well.
Cons:
No Power Boost (has Aim instead). Of course Fire doesn't have Power Boost either but this is worth thinking about if you want to make the most of your control powers. With Fire you are going for all out damage.
Slow to build domination - slowest of all assault sets. This is less true in PvP where hitting your target is more important and difficult than having a super-fast attack chain. It is noticeable though - fire is faster to build domination, not quite as insanely fast as the machine gun of Psi but still faster. Note that "slowest" doesn't mean "slow" - it's just slower than the other sets.
-Def is nice but not great. having -Def is useful but in general will not make as much of a difference as -Recharge does in say Ice or Psi, or the stuns in Energy simply because your accuracy is likely to be very high already. This means playing either Fire or Thorns is more dangerous since neither of them has defensive secondary effects in their attacks.
I've only tried Fire at low level but I liked it. It's fast, and the damage is very good. I think there are only two downsides to it - one, no secondary effects - it's all damage, two, it has a snipe which with my playstyle wouldn't fit.
I think Fire/Fire would work in PvP if you know how to handle yourself.
Filth! Welcome back! 
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I think Fire/Fire would work in PvP if you know how to handle yourself.
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Well all I know about it come from dueling one fire/something at lvl 50, I think the best it did was survive 4 hits. Other times it was 2 or 3-shotted by my blaster straight after spawning..
I reckon fire is more damaging one, but damage alone just doesnt help much in pvp. Repel from singularity might even have some use.
ps. theres no such thing as brawl index. People refer to damage modifier now.
Interestingly this is relevant to me as I've just "discovered" the Dominator AT, and am well into it. What got me into it was a Plant/Thorn I rolled, and while I loved the Plant side, and the Thorn side was ok, what I loved more was Domination and getting it up as much as poss. Heard Psi was fastest, but didn't like the idea of Psi much thematically - wanted to wreak havoc with AoE damage. Have just rolled a Plant/Fire, and I have to second the opinion of someone above, it totally rocks. Sort of like being a fire version of an Ice/Ice blaster at this stage (before AoE Hold).
Great AT for me, def
i wonder wich one between these secondary sets are the most damaging.
what are the pro and cons of the two ?