Reliability - Yamaha is arguably more reliable. But there’s no guarantee. Yamaha does operate a lot cooler than most brands, not just Marantz/Denon.
Sound - in Direct/Through mode, they sound the same. They only sound different if you use some form of EQ or DSP.
Regarding Auto EQ - some prefer Yamaha YPAO, some prefer Marantz YPAO. We’ve seen both YPAO and Audyssey produce good FR graphs. If Auto Room EQ is salient to you, then this is where you should focus on. If you are like me and couldn’t care less about any Auto Room EQ, then no big deal here.
Ease of use - Yamaha arguably has better Remote App, WiFi Music Streaming App, and HDMI compatibility. I’ve used the Denon/Marantz Remote App for years and it sucked for years.
Of course, everyone has a different opinion. I am trying to sell my Denon AVR-5308, which is in my living room. I would replace it with a Yamaha, probably a RX-A1080.