haroo john,
Assuming this is for your car... coz the only tom-tom gps I know is a car one. I don't have a car gps but I do have a GPS, fishfinder, radar and autopilot on our boat... all by a company called Simrad. So I can only give you advice from an aesthetic and features point of view - not a brand comparison.
The most important thing is to be able to read the screen in the sun! Take the machine outside and put it on your dash, twist it round and have sun bouncing off it in all weird directions, try it with your sunglasses on... sometimes the screen may have polarisers built into it, so when you put your polarised sunnies on you can't read a damn thing on the screen... very frustrating.
The reason we spent 10 grand on our equipment was for this visibility in the sunlight. No other equipment that I've seen, from digital cameras (that advertise sunlight visibility but don't) to other boat gps systems, stand up to what we've got... so it's pretty important.
Do they talk? We had one in a hire car in France that spoke to us and it was awesome. It didn't have a displayed map but it'd just shout out instructions from time to time. It was really really good!
Ahh can't think of anything else... xcept make sure the menu's are easy. Ya don't wanna be trying to program some complex vcr-like machine while doing 100ks/hr!