But won't you use a lot of data using Google music?
Depends what you mean by a lot. You can store it in Google Music and still save it for offline play on the device. Love not having to have two separate apps to manage my music.
To the OP, I would ask you how much music you have? I wouldn't recommend buying a 64GB card if you have like 4GB of music. In that case, maybe store it on your phone and if you end up needing more space down the line, go buy the card then.
Of course, if you have like 10 or 12GB of music, go get a card now for it. Personally I just use Google Music and stream it all and then I have a few playlists in Google Music stored locally for offline play for when I lose a data connection when I'm driving in rural areas or if I start getting close to my data limit (which really hasn't happened, but I'll be prepared when it does). I probably only have about 1GB of music stored on the phone though.