You really have to be careful with the sept concept. Lots of family names are septs of multiple clans. I would imagine that either clan society would be glad to have you and your wife as members as long as you pay the membership dues. If you really want to do it right, then I would research where in Scotland her family came from, then make your decision based on what you find. Then if you are wanting to purchase a kilt in the clan tartan, make your decision based upon that research. You will be better off for it. If you must have a kilt before that, then there are universal tartans available and district tartans. With the thousands of tartans out there, I'm sure you could find something to tide you over.