I realize that everything always depends on the circumstances surrounding your team and the competition in your division, world etc, but if you've had the same two catchers catching the same pitching staff for the same 4-5 seasons (and assuming they each catch approx the same SPs and RPs over the season) would the simple formula of RC/27 - CERA be a pretty accurate determinant of who performed better?

What other factors might I want to consider?

How much of a difference would be significant?

I started looking at the past few seasons in a very simple spreadsheet and was surprised by what I found. I used the following 6 columns and it took only a few minutes: Season, Player, PC Rating, CERA, RC/27 and RC/27-CERA.

I tend to find that CERA is not a reliable performance indicator.  Even on the same team for several seasons, too many variables take place; one may catch a higher number of games thrown by your aces, catch at a higher number of pitcher friendly parks, seldom be the victim of a 14 run game, etc.  I would take it completely out of any equation.