Do "I guess" and "I would say" have the same meaning?

Solution 1:

They have only very approximately the same meaning, in that both imply some degree of uncertainty and leave room for others or for facts-on-the-ground to contradict what follows.

• I would say characterizes what follows as a personal opinion or judgment: From what I know of him I would say he is coming.

• I guess characterizes what follows as a conjecture or inference: Well, if he were coming he'd be here by now, so I guess he's not coming.

But there's obviously room for some overlap: an opinion may be based on inference, and a conjecture may be based on opinion.