Everyone told me to get a Tom Tom, but having seen about a £100 difference, I bought an S30 Navman as I read in the reviews its great for days out, holidays and general day to day stuff, which it is and all I needed it for, but not if you need it all the time such as a sales rep - I wouldn't know, I've never used a Tom Tom.
I don't know what else you could expect - more detail maybe? More accuracy? More up to date? There's only been a couple of incidences for me where the road layout has changed, but you could tell it had been done very recently. It has also taken me strange ways at times and its told me to go one way but a sign has told me to go another, so I followed the signs.
I have to say, it's most handy for warning you that speed cameras are coming up and what speed you should be doing.
The S30 also includes Ireland which I will need soon as I'm planning to have a trip round Ireland at some point soon. You pay quite a premium I noticed with the other Satnavs for anything other than Britain.