Wow. You sound a lot like me a month ago. We had been shopping for a new toy hauler since after Christmas. After a LOT of researching, it came down to the Fuzion 420, XLR Thunderbolt 420, and the Vengeance 40D12. We really wanted a side patio model and that's what these are.
All were very similar in price and quality. I really like the XLRs but when you upgrade the Vengeance to the Touring Edition models, they are just as nice in my opinion.
I would recommend any of the three I mentioned. But since you're only looking at the Fuzion and the Vengeance, I'd honestly say go with the one you like best. Quality will be very similar with all three. We ended up picking the Vengeance because it had the nicest side patio and every time I saw a post about a Vengeance on the Forest River Forum, the owner was very pleased with the fit, finish, and quality of their camper.
Plus, the Vengeance Touring Edition models have three ACs. The Fuzions have two.
Last note: Ditto to what chiefneon said about the Heartland stuff. Every dealer I talked with had nothing but nightmare things to say about their quality issues lately.