Garmin has the best routing and most accurate maps for North America, and Tom Tom is the leader in Europe. The difference between Garmin and Tom Tom, however, is largely personal preference, as they are both very good on either continent. All of the other brands of GPS units are trying to catch up to Garmin and Tom Tom.
As for which unit model number to buy, that depends on the number and types of bells and whistles you want to pay for. I've only owned Garmin units, but I'm familiar with several. Other than the least expensive Gamin units, the mapping and routing is going to be the same across the board. You just need to make sure the unit is one that comes pre-loaded with NT North America maps. (The cheapest ones won't have Canada, for example.) After that, it's bells and whilstles.
Read over in the Tech Forum for the GPS threads. They're coming out with new units all the time, so a bit of the stuff in the Tech Forum may be dated, but there are some comments about the various bells and whistles, to give you an idea of what they are and whether or not they are important to you. I'd stick to the newest units, as not only do they have more bells and whistes for the money, at all price ranges, but the electronics are better, as well.