The importance for me are as follows

Up to date maps of Uk and Western Europe
Voice instructions that give road names as well.
A large screen that is sunlight readable.
Traffic situ
Easy to use menu's which seem to be excellent with Garmin Nuvi and are considered to be superior to Tom Tom (i.e. designed for the dumb over 60's to use becuase of loss of memory :-))

If there is a something around £100 that does the job, tell me more.