You will find fans of both. Fans of Denon will tell you to get the Denon. Fans of Yamaha will tell you to get the Yamaha. I have the Denon 4300, and I like it very much. What I can tell you is that while it runs warmer than the Pioneer Elite SC67 it replaced, it doesn't run all that hot. As long as you have adequate ventilation, I don't think heat would be an issue.
You like Audyssey, and the Denon is less expensive. I would say go Denon, but again--I am a Denon owner.
Really, you could flip a coin between the two and be happy with either one, I think.
My humble system:
Sony X900e; Sony UBP-X800; Pioneer Elite BDP-62FD universal blu-ray player; Denon 4300H AVR, Dual PSA S1801's; Monitor Audio Silver RX-6 mains, RX center, and RX surrounds; one pair NHT mini Atmos speakers; Home-built HTPC (Xeon E1230, 16gb RAM, Crucial M500 480gb SSD, GeForce 980Ti, Corsair CX600, CoolerMaster mini-ITX case)