Hey Sarah,
Both are great guitars. A celebrity is going to be rarer so you will probably pay more for one then a Ric. but then again Rickenbackers are American made, so will probably not be cheap. the nice thing about Ric's is it's not too hard to fine them on places like ebay. I think it all comes down to your personally taste. it's pretty rare to see a mosrite 12 string with a tremolo, but I have always really wanted one, even though it might end up being a royal pain is the a** to keep in tune, lol.
tonally, a Ric is going to be the closest thing in tone to a mosrite then any other major guitar company, IMHO, so like with everything in the guitar playing world I think it all comes down to personal taste.

good luck on your hunt. if I find one I'll PM ya
