For me it depends on how busy the secondary is. I love my Water/cold and she is pretty damn powerful. I just drop Sleet and start blasting. However on my pistols/pain I feel like I spend far to much healing and buffing and not much free time to blast, and often feel like I would have been better off to have rolled him as a defender.
Now this may just be my perception, but in the endgame content, I feel like the difference between them is Negligible. Once you take into account procs, IOs bonuses, incarnate powers/passives, most toons are pretty tough, and deal more than enough damage. This makes larger buff/debuffs numbers less important, but it also makes more damage fairly pointless too. There's never really been a time where I feel like "if only that defender was a Corruptor" or vice versa.