I had a 13" mbp for 5 years and it was great until the last 6 months. The only problem at the end was it needed more memory. Dual booting with linux was a pain, so I stuck with OSX which is a great OS once you get used to all the key commands and helpers (get used to using spotlight search).

I am on a macbook air now, but I would not suggest it for development. Its very thin (which is nice), but its noticeably slower, and I am worried I will run into the memory cap quickly.

The funny thing is that I have been looking at Dell's 13" XPS Developer Edition. Has anyone tried it?