As an emergency backup for both, I use a Garmin eTrex Legend Cx. With the MapSource World Map on the micro SD card, I can generate often-used overwater routes. With the North America database on a 2 GB card, I can get most of the US on it. The Legend has a street-following routine, but it is obviously not as robust as a purpose-built car navigator like the Nuvi. Then again, the Nuvi is close to useless in the cockpit...
Once you get your World and area CDs and the SD cards, other ROM updates are free. I suppose you could update the databases regularly, but I don't think it's cost-effective for the purpose... Shop around for software buys.