I've had a Garmin StreetPilot III for a few years. I have tried a couple of factory-fit GPS's, but I would say they didn't come close to the performance of the Garmin.
The biggest difference in quality between GPS in my experience is user annoyance. Seriously. For instance, the Toyota Avensis GPS has a tendency of taking you off the motorway, a couple of miles down the road, round a roundabout and then back on to the motorway AT THE SAME JUNCTION!!! And if you decide "Hey, I know that turning left here isn't the quickest way", it won't recalculate it's route but will instead insist you turn back to where you left off. After a few minutes I was contemplating driving the car into a tree just in the hope it would turn the GPS off
But, my Garmin has never done that, and recalculates routes on the fly. The only problem I have had with it was when I tapped in the address, was given several options and picked the wrong postcode.
Ended up at the wrong end of Bristol.
Looking at the features of the one you suggest, compared to the similarly-priced Garmin... I would choose the Garmin. Reasons being:
1) It includes maps of Europe as well (the Navman only includes UK & Eire)
2) The Navman groups houses in bunches of 60, but my Garmin will take you straight to the correct house (and even tells you what number you are currently driving past if you want!)
3) My Garmin (and presumably this one) includes lists of various things such as hotels, hospitals, petrol stations, restaurants and cinemas. Very useful if you are in an unfamiliar place and find yourself hungry/in need of entertainment/very very sick.
I believe all of them will show you current speed (Garmin is to an accuracy of 0.1mph and resolution of 3 s.f.) and have a rudimentary trip computer. My Garmin includes
Total driving time
Moving driving time
Max speed (hehe)
Average speed
Average driving speed
Total distance
Presumably the new Garmins include the same stuff as my old one, plus more.
I think you better try them out, and see what you think before shelling out.