MP3Pro is different than MP3. With pro, you get the quality of a 128 mp3 in half the space (64). I wouldn't personally go the pro route since it is a liscensable technology. Therefore, many hardware players do not and will not support it. If you're worried about space, I'd suggest the ogg vorbis format which is higher quality than mp3 at a smaller bitrate. Also there is already some hardware support and more will follow.
One other suggestion I would like to make, instead of using music match to rip and encode, use Exact Audio Copy and the Lame.