Terry is correct, the SEL adds 4.5 inches to the rear-seat knee room. This adds the same 4.5 inches to the rear door dimension, wheelbase, and overall length. That is the only difference, other than of course the differences in spring rates, driveshaft length, weight, etc.
My Service manual indicates approx 185 pounds difference in comparably-equipped cars.

I own the SE, and actually the rear knee room is not so great, less than in today's E-class. If you are over six-feet tall, the person behind you will be kinda cramped.
Asthetically, The SE has a clean, lighter, more modern look, but the SEL looks more limo-like.

DG