First, let me correct an earlier entry. The III+ does come with an external antenna. One can remove and replace this with an active antenna if they'd like. A good active antenna that is optimized for gain and low power  consumption (remember - without a 12v cigarette lighter adapter, you're at  the mercy of 4 AA alkalines!)will cost around $55.00 - not $100!  This is  my second Garmin, my first was a III model. I upgraded to the III+ as the  base map is more detailed. For example, on major highways, it'll tell you  what services are available, if any, at the exits. These  "services" can be further defined: another screen will tell you  the names of the places! And this is standard equipment! Various CD's are  available that allow one to  upload more detailed street-level maps of most  of the cities and/or towns in the US.  CD World Atlas and Topographical  maps are available as well. I've used the unit in the car and on aircraft  and have always found it to be incredibly accurate. Battery life is good,  even with the active antenna attached. The only improvement I'd desire  would be a rechargeable NiCad battery pack!