MuseScore is, as it states on the website, "free music composition and notation software". Now the most remarkable difference is, that Garageband is a tool made to have fun and fooling around with different instruments. Where as MuseScore is focused on the notation of music.
Yes, you can create music in either of the software, but MuseScore and Garageband are quite different.
Also, as far as I know, you can't create sheet music in Garageband - only intrumentals
