He is a very good keeper but he isn't too hot with his feet and he has far too much to say for and about himself for Arteta's liking. I suspect the 'values' were an issue in him staying with us.
Despite current form, I would think Raya is the slightly better keeper. Both have exceptional handling, Raya is better with his feet. Martinez is more aggressive and full of himself (in a positive sense).
Villa and Anfield are going to be incredibly difficult games.