When you're comparing a Garmin, Magellan or Rand McNally all three of those companies get their maps from the same third party source, NAVTEQ. The only difference in the maps between those three companies is going to be how current they are based on their map update policies. Garmin updates maps quarterly. Magellan updates maps every 6 months and Rand McNally updates maps once per year. Each company does put their special touches on top of the map data. For example Garmin has photo real graphics for freeway interchanges while Rand McNally has a database of construction and dangerous curves and other things that they build on top as well.
Completely counter to the frequency of map updates, I find in my personal opinion that the amount of features that the GPS has is exactly the opposite order. The Rand McNally devices are loaded with features followed by the Magellan, with Garmin having the least amount of customized features. As an example, Garmin units technically have a feature that tells you when you are exceeding the speed limit. It does this by turning the speed limit graphic on the screen red when you exceed the speed limit by 1 mile per hour and it does this silently. In my opinion that is completely useless. The rand McNally allows you to set the threshold by which you are interested in knowing that you have exceeded the speed limit and when you do it gives you a voice alert that says speed warning. There are many examples of this but whether those things are important to you or not is a personal decision.
From a usability perspective and how refined the products look my opinion is that the order is the same as the map updates with Garmin being the best, followed by Magellan and then Rand McNally. The selection of the GPS is all about trade offs and compromises and your personal needs. That is why we have choice.