I'd only ever go Tom Tom but its just because I know and like the brand.
From what I can gather you have the following:
One - local is just UK maps, Europe is.... well, Europe too!
510 - Has European maps already, speed camera locations pre-loaded and can link in with blurtooth phones for a handsfree solution which I'm told is crap. It can also control your Ipod and link in with a traffic module that gets up to date road info over the radio but I haven't seen this work so can't comment on how good it is.
910 - No different from the 510 except it has USA and Canada maps in case you happen to get on the wrong ferry and has more none sat-nav features such as MP3 player, "text to voice" (plays back text messages when linked to your phone and can read aloud which road you are on) and "wipe your arse" feature for when you just don't have time to stop and do it yourself.
OK.... so I'm clearly not impressed by the 910 but I did recomend my housemate to get the 510 traffic becasue it was actually cheaper than the 510 standard and she is pleased with it. Personally, if I was getting one now (mine got nicked a while back!!) I'd get the tomtom one local and get European maps from another source
