Make your decision based on the resolution of the video, not what format it's in. MP4 is generally used to encode high def video and will usually be higher res than WMV or divx and chances are pretty good that your MP4 vids are higher quality than your WMVs. So just check the properties of each video and keep the ones that are in the higher resolution.
If you're planning to convert videos you already have to either format, then that's an entirely different subject.