I believe the DVD-2910 is actually the newer version of the DVD-2200, not the DVD-2900. The 3910 would be the 2005 version of the 2900 and 3800 players.
The big difference between the 2200/2910 and the 2900/3910 is the audio side, the 2900/3910 are much better audio players than the 2200/2910. On the video side, the 2200 and 2900 have the Silicon Image deinterlacer and Mistsubishi MPEG decoder, so they don't suffer from macroblocking and the chroma bug is not visiable on any but a few DVDs. The 2910/3910 use the Faroudja FL2310 deinterlacer and ESS MPEG decoder. They don't have the chroma bug, but do show macroblocking on some displays.
If you're going for an audio player a used 2900 would be the better choice, IMO. If you're primariliy interested in video performance then the 2900 is still a little better than the 2910, IMO. You can check the Benchmark results for the various Denon players
here.