The problem is not the "quality" of the camper any more. It's quit obvious both campers have survived the test of time by now. So, either one would serve you well.
The thing that REALLY needs to be considered is how well the previous owner took care of it. If it's broken down ... run! You're asking for long-term trouble.
Most important is to look for water intrusion, namely the roof. If the previous owner did not take care of the roof, you'll be able to tell that immediately. Check the roof for cracks in the caulking on all the seems, around vents, and skylight. If the roof looks good, the more than likely the original owner took care of everything else. If the roof is bad, well, ... you've got problems.
There are those who say Keystone is a lousy product. But I've been the owner of 2 Keystone Travel Trailers now, and I've had no heartburn whatsoever. Although I am meticulous in care and maintenance, and if something needed fixing, I fixed it immediately.
With the age of both camper, it's no longer a matter of the manufacturer's quality. It's now all about how well the camper was maintained up to this point in time.