What is the difference between the older GPS-500 and the newly introduced USB GPS stick?
GPS-500 units are based on the very good SiRFstarIII chip. GPS-500 is a very good receiver. Perhaps the only two bad things one could say: 1) The physical design could be improved. Some users managed to break it. 2) It is small, so the internal antenna is also small. That leads to slightly worse sensitivity to weak satellite signals in comparison to GPS receivers based on the same chip, but with larger antennas. But for users looking for a small size USB GPS receiver, the slightly lower performance may be secondary.
u-blox 5 is technology newer than SiRFstarIII. Judging purely by the numbers it seems the chip allows for a slightly better sensitivity to weak satellite signals: -160 dBm for u-blox 5 chip versus -159 dBm for SiRFstarIII chip. 1dB is not much, it's not something most of us would likely notice, but it is an improvement nevertheless . But there are other factors to consider, such as design, antenna size, etc. The physical design looks better, and I would guess users will not break this one easily. Is the physical size of the antenna the same as GPS-500, or smaller? I don't know. Users will need to test the new USB GPS stick in real life to find out how well it performs.