The search function works real well
Both the SP2610 and SP2620 come with City Navigator mapping
You do not want a SP2620 as it comes with all mapping loaded on a microdrive.
Microdrives are known to fail in high vibration environments (like motorcycles).
The SP2620 does not have a mapping CD that you can load onto your PC (the SP2610 does).
All 3 Garmins mentioned are waterproof to IPX7
Touch screen is easiest to enter addresses
Uses CompactFlash cards which can be bought in large sizes cheaply
No battery for off bike use
GPSmap 276C advantages;
Best screen on the planet
Lots of navigation features
Proprietary memory with max size of 256 MB
If you purchase a SP2610 then price in a 2 GB CF card and bike mount up front
If you purchase a GPSmap 276C then the package is only basic and you should price in City Select mapping, 256 MB memory card and bike mount