The '83 should have a 3.07 rear end, as opposed to the '85, which has a 2.83 or so rear end. This means the '83 will accelerate somewhat faster, but it also means that the '83 wil be turning at a higher RPM on the freeway, and fuel mileage will likely suffer slightly as a result.
That's the biggest difference I can think of...
Hope that helps...good luck!