Go affliction with intensity (if you really want to AoE), master summoning / fel conc (if you want versatility), or simple shadowburn and demonic embrace/improved healthstone (nice bits of both, but no easy aoe or quick cheap summons), for duo'ing with your priest.
Extra range, important CoEx snare (controls fear and scream better), useable drains. Suppression also greatly helps fears, curses and dots, against higher level enemies, and enables use of death coil.
MD/Ruin is for end-game 60+ PvE/AV. Doesn't come into its own without tons of +shadow damage equipment.
Instant corruption will serve your dps a lot more throughout 1-60. Improved Life tap (optional, but very nice) and Dark pact greatly reduces the daily grind (mid battle), allowing your duo to achieve longer and tougher battles than normally possible.
Until 60+, your damage chokepoint is mana (and health). Improved life tap and Dark pact mitigate this. Destruction tree isn't attractive until you late in the tree with improved immolate/emberstorm and conflagorate. Improved searing pain is so so, but you still don't have the equipment to back up the +crit. Ruin, again, is not greatly useful until you have +damage items to capitalize on it. Mana will be your damage chokepoint.
There is also the matter of pets. End-game, one point of improved firebolt (second point does not measurably reduce casting time) and 3 points of improved imp gives a 99dps chain-gun. I've used this combination before, and he's a deadly little turret while he has mana.
Go affliction. Respec somewhere else when you have 51 talent points and equipment to utilize other builds.
Extra range, important CoEx snare (controls fear and scream better), useable drains. Suppression also greatly helps fears, curses and dots, against higher level enemies, and enables use of death coil.
MD/Ruin is for end-game 60+ PvE/AV. Doesn't come into its own without tons of +shadow damage equipment.
Instant corruption will serve your dps a lot more throughout 1-60. Improved Life tap (optional, but very nice) and Dark pact greatly reduces the daily grind (mid battle), allowing your duo to achieve longer and tougher battles than normally possible.
Until 60+, your damage chokepoint is mana (and health). Improved life tap and Dark pact mitigate this. Destruction tree isn't attractive until you late in the tree with improved immolate/emberstorm and conflagorate. Improved searing pain is so so, but you still don't have the equipment to back up the +crit. Ruin, again, is not greatly useful until you have +damage items to capitalize on it. Mana will be your damage chokepoint.
There is also the matter of pets. End-game, one point of improved firebolt (second point does not measurably reduce casting time) and 3 points of improved imp gives a 99dps chain-gun. I've used this combination before, and he's a deadly little turret while he has mana.
Go affliction. Respec somewhere else when you have 51 talent points and equipment to utilize other builds.