Yamaha. Excellent value. Also, suggest you call a local dealer's service dept. that handles both brands, and discover service experience of both, AND the quality of support that they get from each manufacturer.
You should be able to listen to both of them. They each have their own sound. You will like one better than the other. They are both good. Try to use your own speakers. Maybe take them to the dealer to be hooked up for listening or borrow the receiver from the dealer for a home trial (even better). If you plan to mail/online order then you are taking a chance that the opinion that you finally accept will match your own tastes. Good Luck
Can I have a third choice? I've sold both. Denon has a smoother amp, uses better transformers. Yamaha has more digital bells & w., more sens. tuners, better ergonomics. Reccommended Yam to a friend, it died 6 years later. Looked at Marantz? Adcom? Rotel? NAD? I doubt NAd or Mar. would be that much more, especially used. I've seen nice 2 zone rotels for 800 on audiogon. Blow Yam & Den out of the water. PS- youll never use those digital (fake) settings after you show off 2 friends, so spend the money on good amp