I just looked quickly on Magellan's website. The Explorist 300 is extremely limited and only has an 8MB basemap which is pretty tiny. The specs say you can't upload maps to it. So this unit would be OK for plotting a track of where you're been (sometimes called a "cookie trail") but you wouldn't see any useful maps of roads, except very major ones like the Garden State Parkway, etc.
The Explorist 400 allows you to load maps through a USB port and SD memory card and would be much better. Of course you must pay extra for the maps and decide whether you want the topo or city versions.
Look back in this forum for my impressions of the Garmin vs Magellan maps and software. I sold my Magellan Meridian Gold (to another member here!) because I like my two Garmin GPS'es much more. The Magellan's are well made and they work very well - don't get me wrong. I just like the Garmin maps and other features better. Also, consider whether it's worth the extra money for a color screen. I personally find this really worthwhile when it comes to the topo maps.