But it is -4 penalty to get that AC20 or -2 to get 19 in your examples.
I usually have the pally take plate since he has it without a feat, but I hardly use Heavy shield over light shield to avoid the -4 penalty to most of the skills alot that are dex based and my paladin’s tend to have very low dex.
The fighters I make mostly have Scale for no check penalty and a light shield again for no penalty. I don’t tend to see them get hit all that much more often with their 18 instead of 17 or 20. This also allows my fighter to succeed on some checks and still keep a feat.
I do tend to design by concept more and sometimes I have a plate wearing fighter with a heavy shield, but IMO for bang for buck I dont think the +1 ac is worth -2penalty for either the scale to plate or the light shield to heavy shield. Especially for fighters who have to spend a feat for it. The shield does provide that +2 to reflex too which makes it have a little more bang for your buck for upgrading.
If the campaign I was in didn’t see me using my skills that much or using them for the new skill challenge rules then maybe I would do it.