All three specs are performing at a similar level in terms of end game DPS so it really is down to personal preference. Sorry if I ramble on...
Arguably affliction is the best on certain raid encounters and in certain situations that require mobility and positioning (as DoTs can continue to tick whilst you move). Affliction can be boring at time (keeping DoTs up etc) and most of the time you will be finding yourself spamming Malefic Grasp tbh.
Demonology is better in a "cleave type" encounter due to the amount of AoE we can pull out both passively and with CDs (such as pet fel/wrathstorm and Hellfire Aura). Demonology is a fun spec to play around with beacuse of Metamorphasis being a CD that effectively changes pretty much every spell you would use in a rotation.
Destruction is really easy to play and you will probably see yourself at the top of the DPS charts if you reforge correctly and are able to get a constant flow of getting burning embers and using Chaos Bolt with CDs, the only thing with Destro is that you will be oom a lot and sometimes there will be a deadzone where you can't cast anything until you've generated enough mana. We can't use Life Tap in destruction so we have a passive 600% mana regeneration which is sweet but it means you have to be sensible with some spells such as Felbolt to renew Immolate.
I hope this has helped you choose and if you have any questions let me know here on online, in the guild or w/e. Thanks :)
affli and demo got best dps, demo got better burn phase, affli got better execution phase.
destro is good, but scales very bad, maybe 5.1 adding conflag to ember buildup will fix it a little, but atm immolate is too weak and pulls haste+crit down a lot.
if immolate hit every second, it could make it much more viable spec.
Imps help massively and the felguard just roflstomps everything.