I will say about Automobile.
(my opionion) for each major features you like.

Screen. High resolution ==> Nuvi 37xx / 34xx
Easy to use alternative firmware ==> 3750US (can go to every Asian fw including 34xx / but no bluetooth)
Screen + BT ==> 3760US (but can't go to every Asian fw / still can go to 34xx)
Need to use new technology ==> 34xx (digital traffic etc)
High resolution with big screen 5 inches ==> nuvi 35xx.

Just need basic features (low price) ==> nuvi 30/40/50