I am using both TeleNav 5.5 (aka AT&T Navigator) and the next generation (not yet released) of Garmin. Both are good, Garmin slightly simpler, TeleNav slightly more feature-packed. If your purchase includes an upgrade (I think TeleNav in the wild is at 5.1), I might go for TeleNav, though either is quite good. The new version of Garmin is a lot like Nuvi on a handset. And you might want to check into your total cost of operation - in 5 years, after chaging handsets twice, what will either one end up costing you?
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