Garmin GPS Gamin Nuvi. Garmin Portable. GolfLogix GPS.

Am I Better Off Buying New Maps Or Selling My Old Gps And Buying A New One With New Features Every 2 Years?

I picked a refurbished garmin c330 with a 2 year warranty.
I think I would only want to upgrade the maps every 3 years.
Am I better off just getting a new GPS and selling the old one on ebay. I figure that it wont cost much more and I would be getting new features as well.
How often would you upgrade? Every 2 or 3 or 4 years? I use it maybe once a month and one big trip a year.