The GPS signals will all be much the same, much like a cdma or gsm carrier signal, where things start to differ is in the NMEA sentance handling / output to human vis software. Im still very new to the whole GPS thing and Im on as big a learning curve as you.
My guess is it would be a lot like you see mobile phone handset companies competing to push the edge of hardware / software features and functionality.
As trillion pointed out the signals are the same.
Differnces will be visible in the hardware and software.
There are only two sources of map data. So now it comes down to how each GPS software package impliments them. What details to show, how often do they get up dates from the map sources. How old are the updates you're using now, and of course, what features do they execute well.
Opinions will vary greatly though few people complain about iGuidance or it's close cousin Routis. iGuidance is touchscreen friendly but some have noted issues w/ font size for streetnames. -- some recent help has addressed that issue now.
Lots of information out there for the various GPS softwares plenty of reviews. So, take some time to work through some reading on them.