They're about even with the gloves. Neither is great, both can catch.
Bairstow is undoubtedly the better outfielder, this much is true.
But I suspcet the real rationale is that Buttler is just more Southern and posh than Bairstow - the selectors want him in the side come what may and, in the everyone available strongest one day side, there is only one spot - the one with the gloves on (Bell, Cook, Pietersen, Trott and Morgan being nailed on for the batting slots despite Root's performances over the last couple of months).