IMHO, honestly, I would go with whichever is the car in better shape...
From what I've understood, the 616/617 can handle being mistreated more than the 603. If you take care of it (or the previous owner(s) have - get records...), then it should be a moot point, unless you plan on buying it, driving it into the ground, and then go look for your next car. I had a feeling that my 616 had been mistreated a little, but it was still pretty strong - despite wanting to blow the oil fill cap off if you loosened it (!). The odometer was broken, so I never really learned the mileage on it. If you neglect a 603, well, you might not have the same luck. A poor cooling system, #14 head, and an inattentive-to-the-temperature-gauge driver could spell a costly mistake.
If the A/C works and sunroof work on the SDL, that price is reasonable. When you look at repair / maintenance records, certain things can tell you how much an owner cared about the vehicle. Little bits of suspension rubber might be a good tip-off - it was for me.
