"Which is the best!, now that's a loaded question.
From a simple mapping point of view, the garmin uses navtaq mapping which is regarded as superior to the tele atlas mapping used by tom tom. so the "experts say".
My advice would be to buy the latest type with traffic info build in. [tmc] Whilst its not perfect the number of times I have found that it saved me becoming part of a traffic jam, has made it something I would not be without now.