OS X is kinda not linux: wiki
the sleep feature is super fast for several reasons- macs keep much of the hardware "alive", (more like a screensaver on XP than Standby) in sleep mode, and OS X has several features like read-ahead caching and the 'journaling' filesystem that make it easy to get back on its feet.
however, starting up cold, a modern XP and a modern Mac machine aren't all that different in regards to boot speeds.
for boot speeds, much depends on the hardware, and more depends on what software is trying to load when your OS loads. XP can seem to take a long time as it loads drivers, services, and programs that may all bump around and get in each other's way- Macs have nice tidy boot sequences, and are not as easy to bung up with junk.
also, OSx is a *dog* compared to older apples on boot speed. apple iie booted in a minute, on hardware that wouldn't run a graphing calculator today. and I can boot DOS 6.2 is 5 seconds after POST, so there you are :)