Will they pick Prior for who he is or pick Buttler for his batting,
If I was selecting the side, I'd go for Prior if he's fit enough. A batsman-keeper like Buttler is acceptable for a T20, but not for Test cricket where catches win matches and there's a strong consensus that Buttler isn't up to it, and it's not like Prior's batting is a major disaster either, he did get a strong fighting fifty against the run of play in the last Test series* they played. Prior is a good leader within the team too.
What about Jonny Bairstow? He was the man with the gloves the last time the First XI played* and he did fairly well with 10 dismissals out of 32 wickets taken, the same number Prior took in the first three matches out of 45 wickets. His batting wasn't too bad either, he made double figures in three out of his four innings - a feat not achieved by Cook, Bell and Pietersen in those matches.
then you've got Monty Panesar's discipline problems & bring in Tredwell as main spinner?
Don't pick any specialist spinner, England doesn't have one worthy of picking against Sri Lanka instead of a fourth seamer. Joe Root will be a bit of use as a part-time option to break up the routine, something Cook should have done a bit more often in the last Test series* once it became clear he was just as good (i.e. not very good) as the specialist spinners.
The biggest problem for England is the batting. There are two who are certain (Cook and Bell), one who should get in (Root) and then three more places to fill, but with England's best batsman (Pietersen) not in the running for any of them. If I was selecting the England side, I'd go for:
Cook
Sam Robson
Bell
Root
Ballance
Morgan (playing in a Michael Hussey role)
Prior/Bairstow
Stokes
Broad
Anderson
Jordan
* just in case anyone's forgotten, that was the one where Australia won all five Tests, defeated six players and brought down the coach.