Feature for feature, PG blows MySQL out of the water. PG gives you way more tools to use in your queries, indexes etc. to solve app problems.
The real knowledge gap would come into things like all the various tactics you need to learn to get MySQL to produce good plans for your queries. PG has a much smarter optimizer which is harder to control - if it does the wrong thing, it's harder to encourage it to do the right thing, and philosophically they don't support query hints. OTOH, worst case in PG is usually much better than worst case on MySQL.
The real knowledge gap would come into things like all the various tactics you need to learn to get MySQL to produce good plans for your queries. PG has a much smarter optimizer which is harder to control - if it does the wrong thing, it's harder to encourage it to do the right thing, and philosophically they don't support query hints. OTOH, worst case in PG is usually much better than worst case on MySQL.