My guild has been doing Kara and heroics for a while now, and I was also in your position a while ago, so I experimented.
I spent 2 weeks as full DM, 2 weeks as full Destro, and 2 weeks as full Affliction. Sure, when clearing trash, destro was nice. But on a boss fight, be prepared to burn some pots, before the boss is at 90%. (I dont like putting extra healing on our healers). I personally hated DM in instances, DM gave me the absolute lowest damage of any of the 3 builds. I also didn't like UA, for this reason: if you spec for UA, you get a 2 second cast dot for roughly 1k damage. A regular sb takes 2.5 seconds and hits for 2k+, and if you dont spec for UA, instead grab ruin, you'll occassionally get the 4k+ shadowbolt. I would rather cast shadowblots for 2k/4k than a dot for 1k in the same time.
This is my build. Basically, the #1 talent I must have is Ruin. the #2 is Dark Pact, the rest sorta falls into place. I grabbed Emp Corruption and Imp CoA because theres a lot of fights where I cant stand still and toss sbs, any extra damage I can get from my dots is very welcome.