Well, the testing I did was hardly anything official. Back last year I ran for about 2 months with multiple GPS units, usually two at at time to better compare them. I've used two Garmins, three TomToms and one Cobra unit. The Cobra unit I didn't like at all. Maps weren't accurate and the interface was goofy. They may or may not have improved things by now, I dunno.
Between the Garmins and the TomTom units, it's six of one, half a dozen of the other. Toss a coin. Some Garmins do things the TomToms don't, and visa versa, mostly cosmetic and personal preferences kinds of things. I did find the Garmins to be a little more accurate with their mapping and routing, but only marginally so.
If you're a fan of the Palm Pilot and other similar gadgets, the TomTom is probably the one you'll prefer because it's made by the same people, they're out of the Netherlands. The Garmin was made by the same people who started the Radio King avionics navigation electronics, and they were the ones who played the major role in the development and implementation of the NAVSTAR satellite system, they're based out of Olathe, KS.
TomTom has the lion's share of the GPS market in Europe, Garmin has it in North America. Toss a coin.
I settled on the Garmin Street Pilot c550, which is essentially the same as the c3xx line, except that the c550 has an MP3 player (which I never use) and it comes with NavTraffic real-time traffic alerts and automatic re-routing for traffic, which I use all time. Man is that handy.
I cannot overstate the significance of the NavTraffic with a GPS unit in the expediting business. The unit is constantly scanning ahead looking for better routing during the entire trip, and when it finds a better route it makes the necessary adjustments. The real-time traffic comes from an FM broadcast signal that the various DOT's collect from road and camera sensors, so when an accident happens or traffic starts accumulating, the Garmin knows about it within seconds, even if it's 100 miles away. If it can re-route you around the delay, it will, and if there's not a quicker routing around it, it will tell you right there on-screen how much of a delay there will be. And it adjusts the delay up or down as you get there. It's very kewl. Not 100% accurate, obviously, as not all locations on the map have traffic sensor tracking, but it's pretty good, and far better than nothing or having to rely on the CB.
A couple of days after that bridge fell in Minneapolis, I had to make a delivery that would have normally taken me to one exit north of the bridge, but the NavTraffic told the Garmin that the road was closed, and it routed me around it and right to where I needed to go. An accident happened less than 10 minutes in front of my in the construction of downtown Milwaukee, closing the exit I needed to take. The Garmin knew about it almost immediately and had me exit sooner, got me to where I needed to be on time.
I used GPS with a laptop for a long time. Being a computer uber geek that was simply the only option I even considered. I mean, even discounting all other rationales, why spend $350, $800, $1000 on a standalone GSP unit when I can spend $100 and use my laptop and have large maps and pretty interfaces with Streets and Trips and DeLorme? That's a slam-dunk no-brainer, right?
Wrong.
These are not squeaky toys by a long shot. Not even close. These are serious electronics that are far, far easier to use than is a laptop, and oh, by the way, what are paper maps?
First and foremost, a laptop or an Atlas, even under the best of circumstances, are both cumbersome and unsafe to use while driving. This is particularly true when being used in "complicated" areas when you need to refer to the map the most, like in congested areas where there will be several quick routing turns one right after the other. That's precisely the time when your attention must be more focused on the road and on others around you, than on a map.
I was positive that I would miss the large screen of the laptop and would not like that teeny tiny way too small screen on the GPS unit. But it turns out that 95% of the stuff displayed on that large screen is never looked at. On the big screen you look at the little car putting along, you look a little bit ahead of it to see where you are going, you look at the time, distance and direction for your next turn, and that's about it. All of that information is right there in 4 square inches right in front of your eyes on the dash with a standalone unit (I have mine mounted on the dash at about 2:00 relative to the steering wheel, so it's nearly in my line of sight while driving, and I don't even have to take my hand on the wheel to punch the screen.)
I still fire up Street and Trips, for use mainly as an Atlas to get an overview of things, use it like an Atlas, as the GPS unit is a routing tool and not an Atlas. With the Garmin, it's difficult to get the miles and routing from one point to another unless you're already sitting at one of those points. It's always from where you are currently, to where you want to go. With Streets and Trips you can set start and end points anywhere you like. If I'm sitting in Columbus, for example, and I want to know the miles and routing between Cincinnati and Kansas City, it's much quicker and easier to do that with Streets and Trips than it is with my Garmin.
With the Garmin, I have to plug in Kansas City, and it routes me from where I am in Columbus to KC, and then I have to tell it that I want to make a stop in Cincinnati, and then it re-routes me through there, and then I have to look at the routing and back out the miles from Columbus to Cincinnati. It works, but it's a kludge, and something like Streets and Trips is much better for that kind of thing.
I haven't had much expedience with any of the units that have come out since last summer. But, for the most part, all of the models within the same categories (road, off-road, marine) from the same manufacturer will have the same maps. The exception seems to be the cheapest models, like the cheaper Nuvis, for example, that have the complete street-level maps for the US, but do not include Canada maps. If you don't go to Canada, that's not a problem, I guess. (Not speaking or reading French, I'd have never found the consignee on the northern edge of Montreal without a GPS unit. Not in a million years.)
So, the difference between the Street Pilot and the Nuvi from Garmin is largely minor features and interfaces. If you get out and walk about a lot, the Nuvi is probably the better choice as it's smaller, thinner and lighter, and gives walking directions. It's (some models) also got an audio book player, currency converter, measurement converter, a few things like that. Since the TomToms I have used are so similar (but again it's been over a year), I have to assume the same within the TomTom lines.
Slow and steady, even in expediting, wins the race - Aesop