I did some highly unscientific testing yesterday and today.
Used a F2P warlock who has decent gear by F2P standards but no epics. Only thing she has that a "pure" F2P wouldn't have is the heirloom neck (unenchanted).
In terms of dps output at the Test Dummy:
1. Destro +Flashover 145 dps
2. Demo +Bilescourge Bombers 134
3. Demo +Demonic Strength 128
4. Demo +Dreadlash 120
5. Affli +Nightfall 112
6. Affli +Drain Soul 94
I couldn't test Affli +Deathbolt at the Test Dummy because this happened:
...which gave me a "dps" of 582.
So I tested Deathbolt in the wild and it was rubbish:
I also tested in it in PvP (just in case hehe) and it was... =surprise= rubbish!
83 dps /cry.
Now look at how Destro does in the same ground:
A respectable 134.
I took a million screenies of how each spell in each spec with each talent broke down, and I can upload if you like, but the brief conclusions I present are, first, the specs and talents are ranked as above (bearing in mind that I've been playing Destro for over a year so I probably didn't do as well in rotations with the other specs, nonetheless I tried my best with each, cast their special abilities on CD etc, and I'm pretty sure the differences are bigger than any caused by my noobishness).
In terms of fun:
- destro good,
- demo good, (loved Bilescourge bombers),
- affli meh (disliked Drain Soul in particular - really slow and ineffective).
Edit: Two things I forgot to add - first, though the tooltip on unstable affliction says you can stack it 5 times, I never managed to have it up more than 3 times at best and more often could only get 0, 1 or 2.
Secondly I didn't bother testing the other two Destro talents because I've tested them in the recent past and Flashover still very definitely wins out.
Second edit: despite all this, I do truly believe you should play whatever you enjoy, so if you love affli + drain soul or deathbolt, don't let me stop you, go out and have fun with it! <3