Loaner phone full restore?

I got a loaner phone today while they fix my sleep/lock button and was told I couldn't do a full computer restore. I was shocked and forgot to ask if this was because I have 30G and the loaner was a 15G or if I'm just not allowed to pick what I put on it. I don't need a bunch of music, but I'd like as much of my phone back as I can get for the next week.