Thank you, JB. Not that much different.
Maybe walks and hits per innings caught is even too limited. I'd be curious to see a stat like "total bases surrendered" - walks, hits, passed balls, wild pitches, HBPs, dropped third strikes, errors, etc. - per innings caught (with assists, pickoffs, double/triple plays and caught stealings accounted for as well). Obviously it would vary widely throughout MLB, but would be a valid comparison for two catchers on the same team, catching the same pitchers backed up by the same fielders.
Another decent comparison stat for two pitchers on the same team would be pitches/out generated. Catchers who call pitches that result in quick outs would help out their pitchers significantly.