A:Answer The best answer is compare, try to listen both brands with movies or music that you like and get your on opinion. Best Buy / Magnolia can help you with that. They have the technical support and the equipment to allow you to compare. Use their quality and excellent support for that.
Second, I had Onkyo / Yamaha / Denon / Sony and I was able to listen Pioneer / Mcintosh / Cambridge / NAD / etc..
When you talk about Yamaha and Denon, I really do not believe that you can really get wrong in each of them. I particulary like more of Yamaha versus Denon, I believe that ( for me ) the Yamaha sound is clear and it has more "punch" for the same money. Like to top of line of Yamaha is RX-A3080 something like 2K versus Denon AVR-X8500H - 4K or even Denon AVR-X6500H - 2.2K. For me, I testing the Denon AVR-X6500H is not the same as Yamaha RX-A3080 and it is cheaper. Yamaha looks like a tank, the quality of its construction looks and "feel" amazing, try to look and compare.
Again, try to listen, if you have a good deal I believe you will be happy with boht, but for me Yamaha is better in general and I like its sound.
I hope this helped.