Using "I would say" also sounds hesitant, and could be thought of as "politer" in a way. If you want to express an opinion and be relatively sure that it won't sound too aggressive, "I would say" is a good way to start.
I wouldn't disagree means a person is inclined to agree, but I won't disagree does, I think, mean something else altogether. What that is, however, I do not know, since I am unfamiliar with that expression. :?