Hmmm, interesting. I personally would go with Dell because I think the only quality products HP makes are super computers and laser printers.
The Dell Axim X50v also gives you a bigger resolution.
480 x 640 resolution at 65,536 colors (VGA)
Which for me is a huge deal. Standard is 240 x 320, so technically you get four times the number of pixels. Makes a big difference for web pages and such.