I have owned both brands.
The Winnebago was a Class C which leaked like the Titanic, despite use of Eternabond and caulking. I bought it when it was 6 years old and I think the damage, although not visible, had started before I owned it. The Winnebago roof design just didn't make sense. Nice fiberglass roof but it had a seam under the drip edge where the roof meets the top of the walls instead of wrapping over the edge.
The 2006 Damon in my signature has been problem and leak free (knock on wood)
I know one is a Class C and one is a Class A but just letting you know about my experience.
You indicate you're looking at rigs about 15 years old. I suspect that if there were going to be problems they would have shown up by now. I would pick the cleanest, best maintained and most problem free one. At that age condition and care will be the main factors as opposed to manufacturer.
Good Luck and safe travels.