The GM chooses what we play usually. For me, my line up after Earthdawn Third Edition Cathay Quest ended spectacularly, has been Shaintar: Rangers Riding Out the Storm (complete), Necessary Evil (in progress, Veteran for my core gaming group), and 50 Fathoms (in progress, Seasoned for new gamers/players) in Savage Worlds. After Earthdawn Kratas Quest Third Edition wrapped up, Ted has run 50 Fathoms (complete), Deadlands: The Flood (complete), Deadlands: The Last Son (in progress) in Savage Worlds and D&D Fifth Edition (started).
Generally the GM is the one who spends the resources and gets the books, maps, minis, cards, Bennies, etc. for play and thus chooses what we are going to do next. It's usually best to do setting pitches before you invest in a new game though to gauge interest.