I would debate that, MoSXS. How often do you work in OS X? I dual-boot OS X Snow Leopard and Windows 7 Ultimate. Between the two, I'd say they're both relatively stable but OS X takes a slight, if only, lead. Depending on what you do, Windows could be argued to be more stable.
I think the fair thing to say is both are very stable and, depending on the software you use and what you're doing, either can be rock solid or very unstable.
As far as capable, that's not true in the slightest, same with powerful. In fact, if you'd like, I can go down the list of things that OS X has built-in that Windows does not. I'm sure there is an equal list on the Windows side but that's not my point. My point is this idea that OS X is not as capable or powerful as Windows is an absurd fallacy from the days when OS X had massive compatibility issues with a number of more-common Windows-specific softwares and functions.
The truth is now, Windows has been known to take cues from OS X. Though, Microsoft has a long history of taking features it's competition has and saying, "Look at this new thing we have! And you can only find our version of it in our products!" Looking at the IE9 feature page is laughable. "You want HTML5 and CSS3? Gotta have IE9."