"I suppose..." and "I think..." are similar but they are not always used the same. When I say "I suppose so" usually it's in response to something I'm not eager to admit or confront. Like, if someone said to me "You should go to the doctor about that." I might say, "I suppose so." "I suppose" in normal usage (in the US at least) is closer to "I guess" than "I think". "I think" is more for situations where you aren't sure of something. Like, "I think I will be busy today." (You aren't sure.) "I suppose I will be busy today" is more like you *know* you will be busy but are somehow resigning yourself to it.

"I think" is also used to express an opinion. "I think you dance too much" is an opinion. "I suppose you dance a lot" is more like an observation or an indirect question based on some initial facts the person has gathered.