I like OSX for most usage. Windows 7 is pretty dang nice though, and better for games. Now that I have Steam for Mac it's okay. OSX is nice because it's low maintenance - you don't have to run antivirus, a separate firewall, spyware cleaner, etc. Chrome + Adthwart and you're good. And maybe Little Snitch (custom blocker firewall similar to ZoneAlarm).
I would definitely recommend plugging a Sandforce-1200 SSD into whatever machine you buy - speeds up the system across the board. Also, if you get a Macbook Pro, buy 8 gigs of RAM for it (aftermarket). OSX benefits from more RAM more than anything. G.Skill has a good kit for $150 (Apple charges $400):
http://www.newegg.com/Product/Produc...82E16820231297
Contrary to what they'd have you believe, the metal unibodys ding and scratch like anything else. The cheapo $999 white plastic Macbook is far more durable, although more scratch-prone. The metal dents though, and it's hard to get dings out! I'd highly recommend getting a plastic shell for the exterior. They are expensive (usually $30 - $50) but they protect the case from dents & scratches, which is important because Macbooks have great resale value, especially shiny ones
http://www.speckproducts.com/macbooks.html