I have a Garmin Nuvi, mostly because I use one (a different model) at work, so I was already familiar with its format. Also, I've read that Garmin has the most complete and current maps for the US (whereas, I believe TomTom has the most up to day European maps).
They're all fairly easy to learn. The main differences are how the maps are displayed, and the size of the screen. Overall, I recommend getting a larger screen if you can afford it, but beyond that, I'd say to go to some place where they have them turned on and you can look at the displays, and just pick the one which seems the most user-friendly to you. I don't recall for certain, but I think we got ours Fry's Electronics, where they had a fairly good assortment we could look at.
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