I'd go with the 15", which is similar to the model I'm typing this on. The 2.2 GHz quad-core i7 is plenty fast, and the extra cores will often provide a speedup

I'd go with the 15", which is similar to the model I'm typing this on. The 2.2 GHz quad-core i7 is plenty fast, and the extra cores will often provide a speedup. You'll also get a discrete GPU, which is worthwhile. The size and portability of the 15" is fine as far as I'm concerned - I doubt there's a practical difference really. Certainly it will fit fine even on a small desk.