They are different. "I would say so." could be said in reply if asked an opinion, for example "Is this right?" - "I would say so". "I was going to say so." is not used as much. It suggests that intent was there to say something, but then it was not said. For example if someone said "Why didn't you tell me ?" you might say, "I was going to say so but I was interrupted."

Not really. "I would say so" is used when you are talking about a hypothetical situation. Wuen you are talking about something that might happen or might not. Or if you are guessing. "Do you think the weather will be nice today?" "I would say so." "I was going to say so" is a way to say that you want to say something but didn't. Often used when someone else says something that you wanted to say. So "i would say so" is for things that migjt happen and "I was going to say so" is for things that you wanted to say but didn't.