I spent ages researching this earlier in the year, as I was going to Croatia on honeymoon and renting a car for 2 weeks, so I needed something I could rely on. I went with Garmin because the general consensus seemed to be that the maps are slightly better at the moment, and it's more configuarable. Tom Tom seems to be more common denominator fare though, so it's probably easier to use for non-techies.
The one I bought in the end was the Garmin Nuvi 760, which is the one-from-the-top I usually go for in gadgets. It has everything most people should need, with UK, Irish and European maps. If you need US maps, I think you'll need a 770. Got it on Dabs for about €300 at the time.
EDIT: Forgot about performance. It served us very well in Croatia within the limited mapping in that area, however I'd recommend always having a map available too, as there are errors, even locally.
HTH,
adam
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