Pricewise the new Mustang is a better deal. The Vista MusicMasters have become somewhat of a collector's item due to their tone, rarity and colors. They now sell used in the $300-400 range, or about $50-$150 more than a brand new Squier Mustang.
Despite some similarities they are very different basses. The MusicMaster has a chunkier neck with a rosewood fretboard, it's heavier than the Mustang, and the pickup has what I would describe as a more aggressive tone.
They're both good basses though. I personally prefer the older MusicMaster.
One advantage on the MusicMaster is that you can occasionally find one cheap in the local classifieds from someone who bought the bass new, kept it in their basement for 15 years and don't know what they have. That's how I got mine. However those opportunities are becoming more and more rare.
Squier MusicMaster Vista Club Member #1