For the record, I'm acutally pretty unimpressed by Spell Pierce against Shops. It used to be reasonable, when you only had Lodestone Golem to worry about, but consider the popular shop decks today, which could easily open with a Lodestone, Metalworker, or Hellkite, none of which make Spell Pierce pleasant to have.
Particularly with Steel Sabotage coming out, I don't see Spell Pierce as an anti-shop card at all - I'd much rather have it in an Oath/TPS meta.
But that's just it, right? It's going to be right in some metagames and in some decks and wrong in others - but it's not obsoleted at all. Spell Pierce is a real, legitimate vintage card. It's not a flagship card like Force of Will, Tinker, or Lodestone Golem, but neither is Duress.
Specifically on Duress vs Spell Pierce? I think you're still going to want Duress or Thoughtseize in a combo deck, particularly one with Swamps. I like Spell Pierce more than Duress in a more controlling deck like Tez. In Gush it's kind of a wash, but I prefer Spell Pierce.
But honestly I'm not too thrilled with either card right now
I wholeheartedly agree. I've been boarding out Spell Pierce against Workshop decks... I found it doesn't counter the things I want it to counter: creatures. And at times when I need it to counter an artifact spell that isn't Lodestone Golem, Karn, Metalwoker, Duplicant, Goblin Welder, Gorilla Shaman or Steel Hellkite, the Workshop player has the needed two extra mana to render my Spell Pierce useless. I'm definitely not a fan of Spell Pierce against Workshop anymore...
Robrecht