For Computer Science, I would get a 13 inch MacBook Pro. The reasoning is this: you will be able to install and run other operating systems in a Virtual Machine. You will be dealing with Linux a lot and possibly Windows Server; maybe even Solaris and more. The MacBook CPU is a little under-powered for running a VM reliably with decent usability. The same can be said for a MacBook Air. But with a MacBook Pro you will have a portable Computer Lab. :-)
13 inch MacBook Pro will offer you a great combination of both power and portability. How do I know this? I am a professional Computer Scientist. :-()
I'd suggest:
CPU == best you can afford, a nice i5 in the MBP would work well (i7 if you can afford the upgrade).
RAM == 8GB should allow for a single VM to relaibly - 16 GB you could use your MBP and have 2 VM's running reliably.
Storage: at least 256GB.
Good luck and hope you find the Mac to best suit your needs.