Hi Walter
I'm currently using Tom Tom mainly due to it having the most free add ons such as speed camera detection, however most of the common ones use the same mapping database, so the main difference is how they present the data. The best advice I can give is go to http://www.pocketgps.co.uk/ this is a site dedicated to Sat Nav and you will get all the info you need, there's also a forum that you can view or even join all free.
Hope this is of help PS, I find the Sat Nav great, but not 100% accurate, I found it trying to take me down a bridleway on the odd occassion