We finally bought a new one last year, after using the parents-in-laws castoff for a while. Smart phones can do much the same thing, but I preferred to get something that was designed to do the job.
Having researched them and bought one, I then got put on the section once or twice that deals with them at work, so I've now had some practice talking about them to customers. Which model is newest takes more time than I can justify, but the higher the number, the newer it usually is. We only sell Tom Tom and Garmin, but those are the two biggest brands, and Tesco's sell only those as well. There is a useful guide here:
http://www.johnlewis.com/buying-guides/which-buying-guide-sat-navs
Your bascially paying for two things - the features and the size. Get the biggest one you reasonably can - its just makes it a lot easier to see the screen. 4in are going to be cheapest, then 5in and finally 6in. We bought a Garmin Nuvi 2597, with a 5in screen (hence the 5 in the model no.) and that works very well. The 6in screens do look a little big, but if thats what you want, go for it - but the 5in do tend to be a bit cheaper and there are more of them. TomToms model numbers follow the same pattern - You have the 40, 50, the 60 and longer numbers tend to mean more features. The letters at the end often mean what maps they have, etc.
Next, you pay for features. So a basic 4.3in screen is going to be about £70 (the Garmin 42LM), whereas the Garmin 2467 (same size screen) is £119. The difference is the features.
The basic ones should have Lane Guidence, which basically tells you which lane you should be in for coming off the motorway, etc. This is actually very useful (got me round Heathrow with relatively little stress), and they will tend to mark the lanes, as well as tell you (you can often tell whose got a sat nav, because everytone changes lanes at about the same time). Most of them also have free lifetimes maps. This is really useful if you've ever used an old sat nav which hasn't been updated, and driven along a road which doesn't exist on the sat nav (I have). Even if they have been updated there are glitches, such as new roundabouts, etc, but its certainly worth having. They will often also have speed camera/speed limit warnings, which can also be handy.
The mid range ones have more features. Voice activation is on ours (although my wife wasn't aware of it when driving, and told it to be quiet, which made our kids fall about laughing), live traffic updates (which can be useful, although sometimes a bit vague - but it will work out alternative routes, which can be very useful), and Realphoto Junction guides - so the pic you see is actually how it looks (I think this might be on some of the lower models too). It also has Bluetooth, so it will link to your Android phone, so in effect its hands free. the high level ones will search on Google for you, but to be honest, I dont get asked about them very often.
According to the blokes who normally work that section, Garmin seems a little more responsive, but there is not a lot in them, and both companies have very similar features. Best idea is to ask people what they've got at work, etc, and then give them a try. Peronally, I'm very happy with the Garmin I bought - it just works. The only problem we had (and the reason the first one went back) was that in setting it up, I opted for the clock to set up manually, rather than on auto. Fine until it switched off the screen in broad daylight. Thought it was a dodgy screen, but after getting a replacement found ou that its because of the clock setting. After letting it do its own thing, its been fine.
Your best value is a 5in screen, and something like the Garmin 55LM is good value
http://www.tesco.com/direct/garmin-...pdates/495-0596.prd?pageLevel=&skuId=495-0596 . The Tom TOM Start 50 is £89 for much the same thing. Its a shame that all the mid level Garmins are 4in, apart from the 2519, but thats £149. The Tom Tom Via
http://www.tesco.com/direct/tomtom-...alling/177-4695.prd?pageLevel=&skuId=177-4695 might be good value, but the Go 50
http://www.tesco.com/direct/tomtom-...pdates/319-4440.prd?pageLevel=&skuId=319-4440 looks the kind of thing your after - all the features, right size and price.
As for mapping - if your going abroad, you might as well buy one with the whole of Europe, but what you will sometimes find is that the one with the Europe maps will be cheaper than the same kind of thing with just UK. If it costs the same or less, but you get more, why not!
You'll get some bits in the box, including the window sucker but the best thing I found was to get a mount which went on the dash low down, so that I can look at the road at the same time. They all work off a micro USB, so a USB power or lighter socket is fine.
You get what you pay for, people are all different, so everyone will have an opinion on what works for them, but as long as its the right size, your half way there. Good luck.
