Okay first, you would have to put "that" after "guess" in both of those sentences. "I guess that.." Or "I would guess that..." The two sentences pretty much mean the same thing, but you would use the 1st one a lot more. You'd only really use the second sentence if someone was like, "do you guess that this is true or false?" " I guess that this is true". You'd only use it if someone told you to guess an answer and you were responding. Sorry if that was confusing!