They are both very good, and they are both quite different. Improvement in what area are you looking for? After many years of using one product, what is it exactly that prompts you to look for something different?
To answer the question about the number of installations allowed is a bit complicated. If you buy the software-only version of Garmin Mobile PC, then you will be able to use it with any NMEA standard GPS receiver, and you will be allowed 1 (one) online activation.
if you buy the software/hardware package that includes Garmin GPS20x receiver, you will be able to install the software on any number of computers, but it will only work with your GPS 20x unit.