So far, Wing IDE gets the nod as the best Python editor I've tried. I had a very productive day with Wing IDE on Windows, and it worked great, with no crashes, and very nice debugging functions.
WIng IDE for MacOS X is an XWindows app. This makes it very slightly un-Mac-like, but it makes up for it by having some nice GUI themes. I haven't yet exercised the Mac version very hard, but it seems quite nice so far.
I'm grateful for all the recommendations!