My job at Halfords is to sell these things so maybe i can help a wee bit.
Personally i would stick to garmin. Dont touch navmans or the navigons, they give you loads of features for your money but they are extremely cheap, even the plastics they use are cheap.
with the tomtoms you get sligthtly more features for your £, but thats only little stuff you might find yourself not using anyway, like the help me function.
garmins dont look as flashy or operate in such a friendly way, but they are still uber easy to use and they are damn near bulletproof.
we get s**t loads of tomtoms back that have gone wonky, menus locking, refusing to start, uk-only satnavs insisting you are in germant, that sort of nonsense.
the big + point with garmins is they just dont seem to go wrong, and if they do, garmin's return policy is soo much better than tomtom's. with tomtom we HAVE to send off the unit if it goes wrong in your warranty period. Even if it has a nail through it, tomtom want to see it for themselves to make sure. 9 times out of 10, on the rare occasion a garmin comes back to us as faulty, we will ring them and they will give us the authorisation to give you a new unit there and then.
one little thing to note is that tomtom are being sneaky as f**k about upgrading maps now. in the past they all came with an sd card slot so you just popped some more memory in there fi you found out you needed europe, and downloaded the new maps. now they have taken all that away on all but the very expensive models, so you cant put more memory in. Basically, if you buy a uk satnav, then find in a few months that you need europe on there, tomtom want you to bin youe £130 sat nav and go buy another £160 europe one. Not very fair really.
Best thing to do would be to nip down to halfords, and ask to have a look at both. have a play with them and see if the ease of use of the tomtom outweighs its reliability issues. You may find you like the way the garmin operates much more.
one final note - mapping wise they both use the same software - navteq - so there is nothign between them on that front.
hope this has helped
