Ok. First. Destruction is design for pvp and for raids. As a 70 lock,i think that demonogology is the best choice. The felguard is almost has powerful as the mage elemental but it last for ever. You get a charge wich is really good vs casters. For raids or instances, affliction would be a great choice because lots of instant dots and unstable affliction.

Finally, warlock is a great class that will never run out of mana and they are awsome. Happy warlocking ( huh )

:)