Caporegime
At the end of the day, the 620 is worth the premium over the 520, especially if you are happy with a refurb.
520/620/720 all have same CPU/GPU. 820 may have older GPU but has higher CPU clock speed and is a noticeably faster device. Trust me.
I'll try to sum them up.
520 - Poor screen & camera. No flash, NFC, or compass. But cheap. Interchangeable covers. Removable battery.
620 - Better screen & camera. Comes with all of above. Slightly smaller battery (removable). Still quite cheap. Interchangeable covers (which also replaces headphone jack).
720 - Even better screen (AMOLED) and decent camera (even better than 820). Comes with all of above + w/l charging option. Big battery (non removable).
820 - Faster device, AMOLED screen. Dual LED flash, meh camera. Interchangeable covers + w/l charging option. Removable battery.
Go to a shop and see how they feel in the hand and what the screens are like (this is an important factor for me when choosing, hence I like the 620). See what features you want, and get the best price.
Done.
The 720 doesn't have an Amoled screen, it's an IPS LCD. Still a great display though. The main advantage it has it's thinner and lighter than the 820
Cheers for all the comments. Having thought about it I'll be going for the 620 most likely. The 820 all things considered doesn't feel enough over the 620. That said I'm going to try out both the 520 now and the 620 and see which feels best.
That said I'd quite like the compass and Here Drive+ on the 620 so I'll most likely be getting this. £155 sim free too. Cheapest I can find the 520 is £105 sim free so I think it's worth the extra £50. I don't think the 820 is worth £200 in comparison though.
The importance of those features can only be judged on a personal basis, but they seem a little trivial to me.
The phone still has a gps so maps works perfectly. Also the lack of flash doesn't seem problematic as all 3 have poor cameras, so you're kind of polishing a turd by using a flash.
Still, we are happy with the results and we'd say that the Lumia 720 produces very pleasant photos.
Although far from the best on the market, the 5 MP camera on the Lumia 620 actually produces photos with pretty solid image quality. The level of resolved detail is great, colors are very pleasant and the dynamic range is decent. The noise is fairly high but the overall result is still really nice.
The Nokia Lumia 520 fares surprisingly well as far as image quality is concerned. The level of detail is great for the 5 MP league, while contrast and dynamic range are pretty good, too. The noise levels are fairly high, though, and the colors have a slightly unpleasant cold cast.