Either one would be good, I'd think. At this price point I think it really comes down to price and features. It looks like neither of the models you linked to has any of the proprietary features (auto-setup & room-EQ) that might represent a difference between brands.
One thing to note is that the THD spec on the Yamaha website
doesn't match what's on the Amazon page (0.9% instead of 0.06%). I can't find a current Denon page for the 391, but as far as I can tell it's the same amplifier section as the 1312/1612. Comparing apples-to-apples, the power difference isn't significant (80W vs. 75W w/ 2ch driven).
Personally I'm partial to Denon (using a 1612 right now), but I've never owned a Yamaha so I can't comment on them beyond the numbers.