I understand these are the top 2 brands.
I don't need a lot of extra features and such, so here's what's important to me (i'm in US)
1) Basic GPS operation (point A to point B, accuracy, points of interest, being able to do a detour, displaying the estimated time of arrival, displaying all the street names that you're passing along the way and not just some, how easy it is to enter an address) ...