Tough call. For pure music I would say go with floor standing speakers but if you want to cover HT as well then a good sub can beat most floor standing movies for LF effects. You know you're going to end up with big speakers AND a sub one day, the question is just which path will you follow to get there
BTW, it's not that "subs are hard to place" it's that "anything producing lots of bass is hard to place". M3 + sub is probably easier to place than floor standing speakers because you can separate the bass from the mains -- put the sub in the best place for bass and put the mains in the best place for everything else. With regular speakers you always usually have to compromise.
