Wow! Thanks for all the responses. You all have given me a lot to think about, especially in regards to higher level math. I will obviously have to research TT more thoroughly. My 3rd grader is not struggling with Horizons, he is doing great! I just thought that since Horizons only goes through grade 6, I should start thinking about what to do after......and then I discovered that TT has level 4 coming out soon, and that it might be easier to change now as opposed to later.
Also, for those that have used Horizons through level 6, or another program that only goes through the elementary grades, what have you chosen for pre-algebra and up? Thanks!