These are both Ruby gems. They watch for filesystem changes. I've looked for resources detailing the aspects of each and I haven't found any that compare/contrast the two. Are these essentially the same thing just different flavors?
Well Guard is a general tool for filesystem changes that can be used to compile compass/sass, whereas compass watch is a standalone tool that watches only compass changes and isn't used for anything else.
So if all you want to watch is compass files, then compass watch should do the trick. If you also want to, for example, compile coffeescript then Guard can be used to do both, but compass watch couldn't.