I wouldn't do it. The 720 has the park-n-go foot brake which is really nice, the 915 doesn't. The 915 is a really stripped down 900. It doesn't have the cross porting, foot brake and a few other things I can't recall right off. The only real things it has that may or may not matter to you is slightly taller rear tires, a little better seat and maybe a few more ponies.
OTH, if you can get the 915 for anywhwere near the same price, there's no real reason not to go that way, but for a grand more I'd stay with the 720.