I misread this post initially and came up with a whole startegy for a multi warlock group - doh!!!. I now want to multi box warlocks

In answer to your question - I would go destruction.

With 5 players targeting mobs the mobs die too quick for your dots to run. On a boss (or elite) then affliction would rock, or if you had a way of having your lock dotting up multiple targets independent of the others then affliction would be good.

However if you are mainly going to burn down single targets, definitely destro.