I'm not sure what it is you want to do. However, U of C is an incredible school and is ranked quite a bit higher than GULC. Perhaps if you live in DC or want to go into politics GULC might be more attractive. But U of C will open more doors for biglaw, academia, and prestigious clerkships. Finally, I would guess the GULC may rescind an offer if your 2nd semester grades are poor, too. Considering how much you post, it seems like you take your education pretty seriously so I doubt that will happen anyways.
He probably wants to be able to make plans and not have an acceptance pulled out from under him because of a bad test. GULC expects a deposit within a few weeks and once that is in, they will not revoke an acceptance.
The drawbacks to the UofC EA is that, I believe, if you are accepted you cannot apply to other schools as a transfer. If you could have sent other applications to other schools, but think you are headed to Chicago only to find your grades less than what you expected, you may not be transferring anywhere.
I guess you are probably right about that, considering the deposit.
But are you sure GULC would not return your deposit and tell you to forget it if you got a 2.0 your second semester? sounds like contracts case... Moreover, I doubt U of C would be quick to rescind an offer based on a poor test. And also I imagine that U of C requires a deposit if accepted ED. Not sure, though.
I do you see your point and it is a good one. Why not wait, right?