Exchange Support Snow Leopard's got it built-in, your copy of Windows 7 doesn't. Freaky but true.
Overall Both Windows 7 and Snow Leopard are designed to be faster, leaner, stronger and more stable than the OSes they're building on. Windows 7 is markedly more responsive, and you simply feel like you're more in control. We'll have to see with Snow Leopard, but if it lives up to Apple's promises, we're definitely looking forward to the performance prowess.
There' s a whole lot that goes into deciding whether you're a Mac or PC, but whatever one you pick, you definitely won't go wrong upgrading your OS this fall.