If we have a girl, we're going with Isobel and love the nickname Izzy. We will be encouraging people to call her that too but I have no doubt that (if we do have a girl!) she would also get Belle at some point in her life. Doesn't bother us either way, it's kind of out of our hands IMO. I had one nickname most of my life from family and friends but when I moved to WA 10 years ago, everyone over here started calling me by a new nickname (another short version of my name) so now I have two common nicknames. I couldn't care less as I like them both!

Ah, I just love the name Isobel. Good luck with you decision.