Here is a link that talks about static and pass to pass accuracy and the different systems. Alot of good information in there for first time buyers, explains the levels of accuracy and some of the different ways that accuracy is measured.
One more link that talks about the comparison of some systems at the University of Illinois.
I know good info is hard to find, but 2nd link is from 2002 - a lot has happened since then! Do not want to bash, just want to make people aware of the timeline - some new GPS companies were still a dream in 2002.