Obviously their is the price difference, but is it really worth it to pay the extra money for the HPX-2000? Also even though the "2000" is more expensive, it lacks variable frame rate.
The 2000 has 1280x720 CCDs and one more P2 slot. You can transfer clips between cards, and you can make subclips, so you can do something like set up a rudimentary playlist on an empty card and do a very crude edit on the spot, and uplink straight from the field. You can also switch between recording and playback modes instantly.
It doesn't have variable frame rates, but it does have 24p. AVC-Intra is going to be big.
But the 500 is more a narrative filmmaker's camera. The 2000 is much more an ENG camera.
The 2000 has many features that the 500 doesn't, including a much more extensive menu system. I think the 2000 has a 12-pole color matrix, whereas the 500 has only the "color temp" and "chroma phase" adjustments, for example.
The only thing the 500 does that the 2000 doesn't, is variable frame rates. Well, that, plus it's half the price!
The 500 appears to me to be about the same sensitivity as the 2000, but maybe 1 stop more dynamic range than the 2000. But the 2000 has a cleaner picture; less noise or better signal processing or something but it definitely looks cleaner.
The biggest thing about the 2000 is the option slot, which the 500 doesn't have. The option slot lets you use the GPS card (which will write the physical location on earth into the metadata for each shot) or the MPEG proxy card (so you can create immediate low-res proxies when recording) or, most likely, that you'd use it for the AVC-Intra codec card.
Other things are the digital super gain, for noiseless pictures in extremely low light situations, and digital zoom (I know, I know, digital zoom sounds like a consumer feature, but for news they demonstrated how it was actually very helpful; an optical doubler cuts the amount of light by 2 stops, but the "digital doubler" doesn't lose any light, it just makes the picture a tad softer/grainier. If you're out shooting a nighttime police raid or something, you may very well prefer that no-light-loss digital zoom). And the ability to pre-edit your clips, make sub-clips, prepare a playlist, and play it back seamlessly in the middle of a live report -- well, that's the kind of thing I would figure a news shooter would go ga-ga over.