When I buy a camera, I want to have lots of choices, and not be restricted to Sony. I think the Palm is the better buy. Palm does only PDAs, and so has a much higher stake in keeping it going. If you care at all about peripherals, like keyboards, modems, GPS receivers, etc., the 505 is a clear winner. Peripheral manufacturers like to build for the standard and for the devices that have the greatest number of extant units.
Palm clearly wins on both counts.
Buy a Palm, and you'll have choices of peripherals. Buy a Sony and you may or may not be able to find that add-on you want. My personal choice is the 505. The 710 screen is definitely better in most respects, but I don't like the restrictive nature of the device. Proprietary Memory Stick, proprietary serial port, their own version of the desktop software, and NO Macintosh support at all.