I had the 2820 (got stolen) and now have zumo. imho, zumo is lots better. only 3 things the 2820 does better...
1. you can program avoidances on the 2820
2. the data fields are customisable
3. you can see the POI's on the moving map screen
ito mounting, general use and esp off road use, the zumo is better.
BTW: this was discussed in length elsewhere on this forum ... do a search.