I've been using Virtual PC for testing, and while I can't compare it with the other others you mentioned, I can give you some pros and cons.
* Quick to install new environents to test on.
* Images are disposable - fed up with one image, throw it away and load a new one.
* Useful for doing quick tests on infrequently used environments (saves you having a PC sat around with Windows 95 on it doing nothing most of the time).
* Slows down the application you're testing, so not good for gauging performance accurately.
* Can behave a little quirkily sometimes (toolbar icons not appearing til you mouse-over them), so can leave you wondering wether bugs are due to the app or the environment.