Is it cheaper to renew your registration and transfer your plates to a different car plus a title transfer or to surrender the old plates and do a new title and registration in IL?

I would keep the plates and transfer to the other car. Either way you will pay more. One of the things that is not clear is that if you are giving up the first car for another car or if you will have two cars. If you are selling the first car don't pay for the new registration and have the new owner pay for it when they buy it. If you are keeping it you will have to pay for the registration and keep the plates because the new car will need both.