I have a 13" MBA of the previous (Core2) generation - very happy with it. I bike commute so the small size & weight were important, and the OSX development environment for Ruby is fairly well evolved. Jeff Atwood has already flogged the benefits of SSD, and I'm a big fan as well - but whatever size suits your data makes sense, I wouldn't over-buy just for the i7 option. The difference between i5 and i7 usually only shows up in very specific workloads that take advantage of the i7's hyperthreading or clock bump. For general development work you won't see a difference, although both i5 and i7 are substantially faster than the Core2.