Here's some info on the Stingray you might find useful. You're not going to have rot with the Stingray stingers. That year and model of Stingray has a fiberglass stringer system. There is no wood below the floor. The floor is 1/2" plywood covered in fiberglass mat and resin, completely sealed on top. A lot of folks are knocking the Stingray because of age but I have a 1997 model and I can guarantee there is no rot in the boat. It's all in how the boats are cared for.
With that said, if the Stingray has been taken care of at all, I could go that route over the Mariah in a heartbeat. I would not bother with the Mariah. I am very familiar with the Stingray brand, and have worked on a couple of Miriah's and have some familiarity with them. I wouldn't want the Mariah from what I've seen. On top of that Mariah is out of business and Stingray is still going strong.
A bit of history of both brands. The lines of the Mariah probably look very similar to the lines of the Stingray. There is a reason for that. The owner of Mariah copied Stingray when they designed their boats. Some of Mariah's models look just like a Stingray.
On a boat that age a good way to tell how much care it was given is look at the upholstery. If it's still in good shape, the boat obviously didn't sit out in the weather a lot. If it's worn, cracked and aged, then the care wasn't the best and probably a good reason to pass on it.