Really depends on how you divide up the market. Mac users will pay a lot for perceived quality, and they will buy upgrades. I'd point you at Wil Shipley's arguments. It might be a lot easier to get $50 from a mac user, or it might be easier to get $5 from 10 windows users. They're just very different. Doing a half assed job on either side isn't going to win you any fans.

But, yeah, if half your money comes from windows, you should really focus on windows, The growth potential there is just staggering.