I owned both Denon 2910 and 3910. These are my subjective opinions, no double blind test you understand.
I think Denon 3910 and Sony 9100ES to be in the same class ( I used hdmi). I think Denon strives to be more like hdtv and has more 'punch' to it, whereas Sony is more filmlike. I saw macroblocking with Denon (on my Samsung 61" DLP TV), but Sony gave the cleanest picture I've seen yet. Sony also has an audio delay ckt which helps. I do see more layer changes on Sony than on Denon. And Denon's remote and set up system is far ahead of Sony. And Denon has an unmatched versatility in terms of connections and settings.
My two cents... but there is an excellent review of this unit at widescreen review.
By the way, one modder explained to me why this player sounds so good. It has a separate clock for analog audio processing that's not dependent on the main dvd clock. That's not true of most dvd players. Anyway, I take it to mean that once you shut off the video circuitry, it behaves just like a stand alone cd/sacd player. Also, finding a player that does bass management on analog out for sacds and cds was hard to say the least. Many Marantz and Pioneer units do not do it, yet this is essential if you have non-full range speakers. Denon and Sony both do it.
Sorry to run on,
Stan