I've developed professionally (like, at a company and stuff) on Linux, Windows, and OSX at my current gig. I do mostly Java and Open Source stuff like Python and Perl w/ Postgres. For me, Linux and OSX are about tied for most productive, with Windows coming in a close second. For the price, I think you'd be best off with Linux.

Having said that, I am a bit of a Mac fanboy, so they can pry my MacBook Pro at my current job from my cold, unemployed fingers. :)