Best sat nav is TomTom. The end, don't let anyone tell you any different.
As for which one, it depends what you need.
TomTom One V3 is the cheapest, at around £119, but doesn't have a SD card slot - meaning if you buy the UK version, you won't be able to upgrade to the euro maps at a later date.
TomTom One XL (I assume you mean GB), is around £149 from PC World, and has the largest screen in the range - even beating the x20 range. HOWEVER be aware, that it may well be too big for your tastes - particularly at night.
TomTom Go 520 is an absolute bargain at around £210 from Halfords Online with instore pickup. I have a vested interest in it - since I have one. Although only has GB maps, it has all the features of the 720 barring the lack of euro maps.
If you can stretch to £200-£210, I would say go for the 520, as the new features not available in the other models are quite cool. Plus it has a better spec, more ram etc.
Regardless of model, make sure you have a look at a program called Tripmaster which will record your movements, and allow you to plot it into google earth. pointless, but cool.