I did a cursory look at the differences, but once I settled on the 2900 I pretty much lost interest in the 3910. There is quite a bit of detail at AVS, but I'll provide some of the big differences that I know about.

The video electronics are the same between the 2910/3910, except the 3910 has DPIC (Denon Pixel Image Correction). I don't know what that is. When I saw them side by side in an A/B on the same set, I could not tell a difference in the PQ.

The 3910 offers the following over the 2910:
- Variable bass management vs. fixed crossover
- Noticeably better audio DACS.
- PIC adjustments (4:3, wide, zoom) on the fly
- IEEE 1394 connection
- Denon Link
-RS-232 port for 3rd party controllers

The 3910 also weighs 17 pounds vs. the 2910 coming in at 12 pounds. That usually means power supply.

The big differences appear to be in audio (which I can verify first hand), connections and flexibility in adjustments. The only advantages the 3910 holds over the 2900 is in connections and slight audio DACs, but (to my eyes) the 2900 PQ is better.

The 2900 audio is also superb. Hope that helps, in a non-technical sort of way.