I think it's pretty difficult for anyone to pick a test wicket keeper right now.
Who honestly gets to watch much county 4 day cricket these days?
We can judge them on stats, but stats tell less of a story when considering someone for test match selection, especialy considering the amount of runs being scored by everyone in the championship over the last few years.
I'm not too certain on any of them, Pothas has the best average and from accounts is a decent keeper but his technique is far from orthodox and may falter at international level. He's also 33 and if he was given a go i'd see his place as being very fragile as he's not going to be improving at the rate Foster and Davies (hopefully) are improving at. Meaning that if he fails early, the selectors won't be too keen on giving him more goes when he's 33, Foster and Davies on the other hadn would be liable to more games as they're younger and can improve in the future.
Foster had been unlucky so far but even though his stats have been good, i'm a bit cirumspect with him at international level. His technique isn't bad, but he reminds me of Read (who scored the same amount of runs at FC level) in that he wouldn't have a hope in hell of batting in the top order or in a tricky situation as his technique is a bit 'light'.
I too used to be anti Nixon as i was sick of people justifying his selection because he shouted behind the stumps. But, with his recent performances which imo have been quite superb i don't think he'd be too bad a choice. If he gets the job it should be for this summer and this summer only, but i wouldn't be displeased if he got a go. His technique isn't flashy, but from watching him bat he makes the most of it and has the ability to play some important innings for his team. His determination and mental fortitude with the bat is what England have been lacking, Nixon may not score significantly more runs than Jones or Read, but i think the runs he does score will be in situations which are more valuable to the team.
In short, i'd probably go for Nixon for this summer as he's the man in possesion then Foster for the winter and then let it be a competition between him and Davies after that.