Personally, on the standalone units plugging into cigarette lighter sockets, garmin or tomtom, there is not alot between them. Either one will do. The key points are:-
1 Map updates - free, chargeable, lifetime subscription, ease of updating.
2 Traffic subscription, free, chargeable, lifetime.
3 The interface, do you like it?
4 Postcodes, can it deal with the WHOLE postcode (probably yes nowadays)
5 Bluetooth, MP3, handsoff connectivity may be important to you.
If you at all intend to take your car across the channel then the maps ought to be european. Ensure that all the countries you are likely to travel to are included. This is only an issue if you go eastwards to the edges of the current EU where coverage can be less than 100%.
Whether it is a tomotom or garmin, it is only as good as the map data and there are 2 companies teleatlas (dutchies) and navteq (yanks). I don't think there is alot between them.
If you come from a bilingual or multilingual family then it needs to work and preferably speak in these languages.
If you travel further afield then you must choose your satnav with care. Not only must you have got the languages sorted, but the mapping too. Maps are sold in packages for markets and if your requirements straddle two areas then your satnav needs to be able to handle this. Mostly nowadays, additional maps can be downloaded onto the device or purchased on plugin SD cards.
Just because there are alot of posts about tomtom do not assume it is the best. walk into a shop after doing your research on the internet and decide for yourself.
If you have more cash to burn and hate the sight of cables dangling about or non oem equipment hanging about the dash then consider a built in unit. Either double DIN fixed screen or single DIN flip up motorised screen.
Just my 2c as usual.