Nancy's right, you will hear 'the other side,' so I may as well be part of that voice. I have both PPC02 and WM03 on separate PPCs and it took me longer to get the WiFi running on WM03 than it did with PPC02. Of course, I suppose you could say I wrote the book on WiFi with the Axim (well, the web page, anyway. See the links in my signature) so maybe I'm not a fair judge.
Besides the zero configuration WiFi claimed for WM03, the version of Pocket Internet Explorer is supposed to be faster on it. It is true but the latest PPC02 ROM upgrade that Dell has released for the X5 Advanced (
R63937.EXE= ROM A04) has a much faster version of PocketIE than the earlier ROMs. Still not quite as fast as WM03 but pretty good.
Other things to consider are the third party applications your wife has on her Axim. A few programs do not work well or at all in WM03. There's a thread
here that talks about these. You may want to check through it before making the decision, too.
Finally, I want to make clear that I think WM03 is a fine OS. I'm just not sure it's worth doing the upgrade.