Run the front and back the same unless you have 2 different rim sizes, which I have no idea about. The tubes cost more, but I haven't had to replace mine at all since I got them, while with regular tubes I'd be replacing them pretty frequently, so I've gotten my money's worth. You can also try tire liners like this http://www.danscomp.com/449000.php?cat=PARTS but I've heard of mixed results with them.