Does the TomTom have a facility to display/enter waypoints via OS grid ref. or Lat/Long? If not, it might prove a bit limiting if you use the unit off tarmac/green laning etc. regularly?
Something like the Garmin Map60 or an Etrex that supports the mapsource mapping software might be better - they are super rugged, run on batteries as well as 12v hardwire (useful on or off the bike), and have simple buttons to enter info (if you use a touch screen GPS, stick a protective film over the screen or you'll scratch it to pieces in sand and mud).
I've used a Garmin GPSII+ (no mapping, but o/s grid etc.) for ages, and now have a Garmin 2610 which has mapping software and Lat/Long readout too - it doesn't display (well, doesn't seem to anyway) the Topo information on the screen in the same way the outdoor units do, but you'd be surprised how many 'green lanes' and unsurfaced roads there are included in the mapping software, especially in Europe and USA.
Hope that helps...