My take that Raya is a better shotstopper comes from league stats over the last couple of years. He clearly outperforms Ramsdale in that department in that time frame. His time with us is so short that it doesn't make much sense to use that as a relible metric To me at least.
My take on the goalkeeping situation is that we bought Raya precisely because Arteta wanted a goalkeeper who is better than Ramsdale in possession. Which Raya is. And it helps us. Clear net positive. Because having a goalkeeper with those attributes is essential to how Arteta envisions his football to be played.
The notion that we should have got someone who is more than a marginal improvement on Ramsdale seems wildly unfair to Ramsdale. Because Ramsdale is really, really good! Any upgrade is likely to be just that, marginal. Unless we would sign Allisson. I don't watch enough football these days to really know, but I would think that finding someone who is both a superb shotstopper AND top class with the ball at his feet is super hard. There can't be too many Allison's around. Even Pep at Man City is content with a goalkeeper who is superb on the ball but with no other stand out qualities.