Yes they do vary enormously in cost, varying from the flat fee/online only Purple Bricks style ones to traditional commission models, ranging from 1%-5% of the purchase price.
In terms of which is the best, don't simply go for the cheapest, it can be a false economy because even if you spend £1000 more in fees on a more expensive agent, if they achieve a £5,000 higher price then you are still well up on the deal overall. However I disagree with PP that Purple Bricks is always a bad idea, nor do I think more expensive necessarily = better. I think it really depends on the type of house you are trying to sell, where it 'sits' in the local market price point (high end, low end etc), your target market and your circumstances (e.g. whether you are trying to get the quickest possible sale or aren't in a rush, whether you are comfortable doing a lot of your own marketing etc).
I sold a property through an online only agent (not Purple Bricks) and it was absolutely fine and saved me £££ compared to using a traditional agent. But it was a modern, easily marketable flat in a popular area suitable for a first time buyer/couple without children/BTL - the kind of buyer that is very comfortable with an online only model. Plus I was willing and able to do viewings and manage the after-offer process etc myself and priced it competitively so it virtually sold itself. It would have been totally different if it was a high end or unusual property (in which case perhaps a more specialist, expensive agent would have been the best call) or a average family house in a slow moving market or whatever.
How to pick, I would invite a selection of agents around to value the property, a mix of traditional and online, not so much for the valuations themselves (you should be able to find a fairly good idea of what your house is worth from sold prices of similar local properties, available on Zoopla or Mouseprice), but to get a feel for what they offer, salesmanship, marketing strategy, frankly how much of a bullsh*tter they are! I would also look online at recent adverts they have placed and look for a firm that offers really high quality photos and advertising materials as this is a big part of what gets people through the door - again price not a defining feature here, some of the more expensive agents round here have terribly lit, weird angled unflattering shots up, whereas I was really impressed by my cheap-as-chips online agent's photographer. But it can go the other way too! At the end of the day I would go with your gut on who seems to have the best local knowledge and drive to sell your house at the best possible price.