Damo,
This is going to be one of those threads where everyone will rave about their "x" brand being better than "y" brand. They are as good as each other. More coin = more functions, bigger screen generally. I (2 satnavs ago), went TomTom (GO710, then GO720, now GO730), based upon Navman holders not being able to hold fast to the windscreen. (Done with an independant road test...).
I went TomTom as "I" thought the display/userability was clearer/easier, Ive been in friends cars with Navmans, (4.3" screens), and I just think the TomTom is a better unit. GO930 is top of the line, but if you dont need all the bells and whistles, (wank), one of the lower models will do fine.
That said, they changed the windscreen mount from the 710 (awesome - click lock) to the 730 (shit - press on clamp), that never used to hold to the screen. However, TomTom listened to the consumer, and bought out a revised, 710-like windscreen mount. That ive had for several months, and it hasnt moved.
But the NavMan V TomTom thing, is a bit like Holden/Ford, Evo/Rex, etc. Just get ya mum to have a look at both, side by side instore, and make a call.
Brent.