The Aera looks to me like a Nuvi with an extra black border around the outside, and about twice as thick. At least, that's comparing my Aera 500 to my dad's Nuvi 755. Otherwise, they look pretty much the same... The automotive software is virtually identical.
Physically, I don't know that you'll find a Nuvi that's the same case size as an Aera. Maybe a Zumo, but I don't know for sure. I remember Garmin's website had the ability to compare gps units side-by-side, you could print out the specs for the Aera and then select a set of Nuvi's and Zumo's and look at the chart it generates.