The big question on these is are they worth the 70-100 difference from the dell 2410. The stand quality is about the same. Its the same panel. I had a hard time deciding...
The Dell U2410 is $100 more expensive, and to be honest I prefer the HP ZR24w. It has better panel uniformity, better color reproduction in the sRGB color space, lower input lag, and better factory calibration.
The U2410 has additional image preprocessing (although this causes input lag), has an HDMI input, has a 10-bit color lookup table (although it is still a 8 bit panel), has deeper blacks, and has a wide gamut screen (although many see this as a negative).
It all depends on why you want the monitor, but I have tried to type out the major pros and cons of each monitor. The HP monitor is much cheaper, but I think that is temporary, as the non-promotional price is about the same price as the Dell monitor.
EDIT: Here is a review that compares the three monitors in question:
http://www.tftcentral.co.uk/reviews/hp_zr24w.htm