MSN is originally designed for instant text messages, only adding the voice and video later and not terribly well.
Skype, on the other hand, was originally made for voice chat, with the text messages mostly meant for telling whoever you were talking to to turn on his speakers/plug in his microphone. Apparently, some part of the base design has made them ideal for video, as their quality on both this and voice is far beyond anything I've seen over MSN.