The Tavern

So on my Minotaur warrior, I have Str 31, shield skill 20, and the following 3 shields

+2 Buckler of Protection (Max enchant 3)
+2 Shield of Zin (+6 str, rPois, rN+) (can not be altered)
+0 Large shield

What do you use to evaluate the 3 shields? How does that change if you happen to have 6 enchant armor spells in inventory?

Just equip the shields and watch what happens to AC, EV and SH. If you use spells, also look as spell success. Generally I maximise AC + EV+SH.
+6 str, rPois, rN+ is a nice buff, and might make the shield worth using.

With 20 Shields skill, the large shield is best choice.

So small complication. Gold dragon scale dropped. I am currently wearing ego plate (+8, +2 dex).

So the plate plus ego shield = 91(45, 26,20) no improvement possible with scrolls
plate plus large shield = 94 (45,24,25) 8 sh improvement possible with scrolls in hand
Dragon plus ego shield = 83 (41,22,20) 8 AC improvement possible with scrolls in hand
Dragon plus large = 86 (41,20,25) 8 improvement possible across AC/SH with scrolls in hand.

Hmmm. And as an aside, does it matter if you use the scrolls for AC or SH no preference?

I would never enchant a shield, AC is much better.

EDIT:Double post deleted.

Definately enchant the dragon scales, If they suit your character. You might well want to replace the shields at some point, but very few armours beat maximally enchanted gold dragon scales.

I would never enchant a shield preferentially, but I will certainly enchant one if armor slots are maxed. This can legitimately happen if you get some +11 chainmail of 3 resistances, +4 slaying, and +inv in orc or something similarly ridiculous.

Typically I will find some good piece of armor and commit to it with enchant scrolls, then do good helmets/boots/gloves (if applicable/not already maxed/non-artifact), then shields last. But often enough shields get a least 1-2 armor scrolls by end game, especially if I'm doing fire dragon scales or lighter. If I don't have the armor already and do have good gloves/helmet/etc I will obviously enchant those immediately.