I always get one set of medium armor and one set of heavy armor from the forge. I wear those until I get to the lower city in act three. That’s when I start finding better armors. But they do last me a good long time! Is the shield so much better that I should consider it instead?
It really depends on your group, imo. The best part of the adamantine armor sets and the shield is that they let the character avoid crits. So in that sense, the armor and the shield serve the same purpose.
For me, it comes down to who's going to wear the armor or carry the shield. Lae'zel is good for the heavy armor, but then who gets the heavy armor helm? I want to split all three items up so that as many party members as possible can avoid taking critical hits.
When it comes to medium armor, again, who's going to wear it? Shadowheart is usually in the luminous armor for me, and I'm reluctant to change that, especially just before going into Act II. But she can easily take the shield.
Same with Gale and Wyll. Both of them can use the shield.
You might, of course, have a party composition/build where it's more appropriate to take two different sets of armor.
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On my current run through, I just decided to go with two scimitars. Probably the worst decision possible for how to use the adamantine forge, but I can't help wanting to shake things up.
It's always Phalar Aluve and Knife of the Undermountain King, and Blood of Lathander and so on and so on. Watch streamers and nine times out of ten you'll see the same gear being used. The more I play the game, the more I think that's a flaw in the design.