I'm the exact opposite, I think people have over stated Steks performances based on some good shot stopping but there's a lot more to Gk than that.
For example, you say he hardly put a foot wrong prior to City but I disagree. For all his good saves vs City he was actually at fault for the goal that cost us the game due to his reluctance to come off his line for a simple cross he should have took. This has been a reoccurring problem all season.
They both have strengths and weaknesses that negate each other. Robles is a very aggressive cat that dominates his box and comes for most things as well as decent with his feet and great in 1 on 1s but a poor shot stopper. Steks a great shot stopper, decent with his feet but doesn't have an aggressive bone in his body and hugs his line.
Both are decent number 2s IMO, neither are number 1s fur a team with aspirations like us st the moment.