I'm not sure what your budget is, but I would honestly get something like the Duet 2 (actually, thats what I DID do when I bought my Macbook Pro and Logic last year). I do a lot of writing that only involves virtual instruments, and it's been great for that.
I have recently replaced it as my main interface, but I've held on to it as it's super portable. Anywho....just my opinion, but have fun making music with whatever you decide!
p.s
If you want to throw down that sort of cash on a Mac Pro, it might be worth holding off until they release new ones for 2013. Thats only if you care about getting something a bit more future proofed.
Take care!