What exactly are you arguing?
As a top order specialist batsman, Sangakkara is outperformed (either by stats or when placed into correct context and whatnot) by names such as Bradman, Headley, Hammond, Tendulkar, Richards, Lara and countless others. He wouldn't make the side as a specialist bat, given the strength of the competition.
Lets assume we're looking at Sangakkara the wicketkeeper-batsman. He isn't familiar with the number 7 position, where he would be batting in an ATG XI, and he averages less while keeping that Gilchrist does. Adding in my personal opinion that Gilchrist was also a superior gloveman, Sangakkara loses out again.
He's a brilliant player, no doubt about it, but a keeper batting at 3 doesn't work for an ATG side's balance. Plus Gilchrist would do more damage coming in at 5/400 than Sanga TBH (and with an ATG top order, 5/400 is a conservative estimate).
On the Flower argument, what was his keeping like? I haven't heard anything that suggests it was up with Gilchrist, conversely I've been given the impression that it was weaker in comparison - nothing concrete at all, just various opinions picked up over long periods of time. And his lack of experience at #7 might count against him as well.
Hopefully this makes sense, I really need some sleep soon..