I have both software, WinDVD Platinum 4.5 and PowerDVD 4. Here are what I found when comparing them:
Picture quality: WinDVD is slightly better than PowerDVD
Sound quality (stereo): WinDVD is slightly better
Sound quality (ac-3): PowerDVD is slightly better
Video features/settings: WinDVD has more
Sound features/settings: WinDVD has more
User interface/playback controls: PowerDVD is better and easier to use
Playlist management: PowerDVD is easier to use
Loading time: PowerDVD loads much faster
To make WinDVD region free, you can use the software WinDVD Tweaker Pro. And to make PowerDVD region free, use either DVD Region Killer or DVD Genie.
The above are just my personal opinions, others might disagree.