It would be helpful if you were more clear about what particular model you are interested in.
I'm assuming you are talking about Garmins. All of the Garmins use GPS but some of them can't use speed/cadence sensors. You can get speed from GPS but having a wheel sensor is going to be more accurate. A speed/cadence sensor is the only other thing that you'd attach to your bike (along with the two magnets the sensor requires).
It's easy to move the head unit between bikes but moving speed/cadence sensors isn't really practical. The Garmins allow different profiles for different bikes (eg, to deal with differences in wheel size). If you want to use the unit on another bike infrequently, you might not care about not having a speed/cadence sensor on the other bike (the GPS speed might be good enough).
By default (i.e., without a wheel sensor), the Garmins use GPS for speed, which can be inaccurate over short periods of time and distance. The 500 and up units allow you to use a wheel-rotation sensor that avoids the GPS inaccuracy.
GPS (basically) just determines location/position (and assigns a time to the location). Speed is determined from the position data by computing the distance between two points and dividing by the difference in time. Since GPS positioning has some error (+-30 feet or so), the speed between determined from points close together will be less accurate (more variable). GPS speed works better when traveling at higher speeds because the points being used are farther apart.
Speed from wheel rotation is very fast and works accurately at distances much less than the normal GPS position error.
Even if you get loss of GPS signal, it's unlikely that you'd get such loss that you couldn't use it for not getting lost.
Last edited by njkayaker; 06-14-13 at 01:32 PM.