No GPS is actually Mac (or PC) compatible, it's the included mapping software and ability to upload/download to the unit. In that sense, It really doesn't matter, as most better units today are pre-loaded. Mine is seen as USB storage, just upload routes/POI's, etc. into the proper folder.
For the included software, Garmin's is Mac compatible as well. I can't speak to Magellan. For combined cage-MC use, the Garmin Zumo 550 is a terrific choice, I use one almost daily, and it's the best GPS I've owned thus far. I generally advise against TomTom if you're in the States, the basemaps & routing algorithms are generally inferior to Garmin's here. If you're in Europe/Asia, then TomTom is the preferred choice, as Garmin's basemaps are not as good as TomTom's there.