Which model are you looking at?
I was looking into the TomTom but I read it was buggy. Some strange one-time registration issue that caused some people a lot of problems.
I had a Navman PDA based Nav. It worked quite well. I upgraded to the ICN510. It has some nicer features but I don’t think it navigates as well.
My buddy also had a Navman and he upgraded to the Garman I-Que. He says it’s much better than the Navman.
One plus for the Navman is that if you ever travel to Europe you can get street level maps for both Eastern and Western. Not sure if that is available for the TomTom.
Good Luck