I have a Garmin III but have been looking at the Explorist 500 or 600. My girlfriend is looking at the 210. I hope this is that right place to ask these questions . First, what is the difference between the 500 an the 600 (other than $50.00) as the specs look identical and is it worth the extra money? Second, my girlfriend wanted to know if the 210 would make her a good first unit? Thanks for the time and bandwidth.