I'd say play SoA alone as a stand up game as it was sold. Then add ToB and play it again. It's certainly best imho to start a new game when applying ToB for the first time. I think that the new additions and abilities encountered in ToB are first class. So are the things in SoA, if you do SoA first you can stand gobsmacked, if you then do ToB you'll be gobsmacked again. If you do them together for your very first time....you'll only be gobsmacked once.