One thing to consider is aoe potential. My current grp is pally, 3 mages and priest and I am aoeing my way through instances. Not used a sheep yet.

I have a warrior, 2 mage, lock, priest group at 60 btw. (on another server) The only thing I might miss is the ss since my priest never gets aggro with nova I would always have plenty time to move him to a safe spot to die. DI sounds trickier to use although I haven't tried yet. I am finding my new group alot faster, easier and more fun than playing "normally" but then I always liked aoeing even when I played one mage so It comes down to what you enjoy doing I spose.

If I had a 61 lock already I'd do mages personally. Then you can go either way later on. If you abandon a mage now will be a pita having to level one up from 23.