UMS and MSC are the same thing. This type of player functions as a mass storage device, and will show up on your computer as an external drive (eg "E:\"), just like a USB key or portable hard drive. You drag and drop your music files -- an application such as Windows Media Player is not required in order to transfer songs. You can put your music into folders if you want to sort it by artist, and these folders will show upon your device. Otherwise, the songs will be listed alphabetically.
With MTP, you will often need to use a program like Media Player in order to transfer files. The format also allows for metadata (album covers, automatic organisation of songs by artist, title and album).
If you have a Mac (and don't want to delve into the world of iPods), you'll want to go for a UMS player, as these should show up as an external drive on your computer. MTP players tend not to be compatible without a third-party program and a lot of fiddling around, and even then it's not guaranteed.