What the other posters wrote are exactly what I was thinking of when I saw your question, but I'd like to add that (personally), I think SL's book choices are better. There is a lot of similar books between the two but somehow, the SL books are more memorable to me.
I LOVE, LOVE, LOVE SL but decided to switch to TOG because of the combining problem. It does take me more time to plan the week's work with TOG than with SL. SL's Instructors' Guide are more user friendly than TOG. You also have to plan to spend time reading the teacher's notes to get ready for the weekly discussion in TOG. (Some weeks, it's easy. Some weeks, it could take a chunk of time.)
I still pine for SL but TOG is working out well for us this year.
God Bless,
Anna