I believe the Denon DVD-5900 has a better transport, beefier power supply, and more solid/heavy build quality which improves damping and noise problems. All these improvements will yield better audio results over the less expensive DVD-3910. The DVD-5900 was once the flagship player from Denon (until the 5910 came out) so a lot of effort was put into this unit. The DVD-3910 is a lower model, although still good, I would still give the edge to the DVD-5900.
Good luck,
Harvey
Good luck,
Harvey