There are several areas where GPS units excel or fail. They are all similar with regard to features, the mapping etc., some use spoken street names, some just turn directions. I have (in different vehicles) both types and don’t really find one superior to the other – as long as I know where to turn I don’t really care what the name of the street is. In fact looking for street signs to confirm that the GPS is indicating the correct street can be a distraction. My biggest issues aside from calculation speed and accuracy are traffic reports and speed compliance (lets you know if you are exceeding the speed in a particular area). The POI (Points of interest) are handy – closest service station, retail or service outlet etc.
The real noticeable differences are in the speed of recalculation (nothing is worse than approaching a potential turn after changing direction (traffic obstruction – missed turn etc.) while the device is “thinking” and not providing direction, and the menu system. Those that have intuitive menus systems are the ones that excel. There are magazines devoted to “road tests” and specification & price comparisons. I suggest that you invest in one.