To update a bit -
The 5900 is indeed better. It addition to a better display, it has (basically) scroll wheel-like functionality where most of the operation is done by turning and pressing a large center scroll wheel (similar to the MP6800 - if you're familiar with that one).
Both the 5900 and 4900 have multiple connection options though - the regular IP-Bus connector, the new iPod-bus connector, and a 1/8" aux in port on the back. I have my iPod, my Sat Radio, and a laptop all connected to the headunit without a big mess of wires and boxes everywhere - it's quite nice.