I own a Streetpilot (2820). It does not have a battery backup.But I have read that other Streetpilots do, so that may not really be a point of distinction.
It has a larger POI database, multi-destination routing, etc. that was not available in the nuvi that was realeased then (nuvi 350). Things may be different now.
I think the biggest selling point of the nuvi is that it is designed for the traveller - lightweight, long battery , you can even use it in walking trips, and comes equipped with currency converter, basic transaltion in a few languages, etc.
Hope this helps