I have the Trek 820, but I can get the Cannondale F9 for about $30 more, I did a google search and it comes up as the Cannondale usually goes for over $100 more than the Trek 820... Is it better, should I trade out for the Cannondale?
dude, I would recommend that you keep the 820. Both bikes are pretty similar and isn't this like the 3rd bike you're on in the last 2 days?
anyway, IMO the F9 and 820 are similar enough that you may as well keep the 820 if you're happy with it. If you're not that happy with it, go ahead and test ride the F9 and switch.
I used to hang around the Yongsan army base about 20 years ago and it was a lot of fun.. There isn't really any need to get a nice mountain bike in Korea (not that many trails, etc.) so I think your 820 will be fine.