The ATI player is actually based on the Cinemaster DVD player, which has better image quality than PowerDVD. However, in the ATI version it is plagued with problems with sound support. While all analog modes are supported and work fine, DD and DTS passthrough via the S/PDIF output is suspect at best. DTS is definitely not supported, while DD is, but I've never gotten it either to work anyway.
PowerDVD XP 4 however works fine out of the box, and while image quality might not be as good out of the box, it can be tweaked to a certain extent to match different displays. Digital sound quality(DD/DTS) is a bit less dynamic than WinDVD 4 for me, but IMHO it beats WinDVD for picture quality. To get the best of all worlds, you can use ZoomPlayer to choose the audio and video filter of your choice, provided you have the other players installed.