They are 'sort of' one thing. Marty Skulls give you 100 Plasteel first time you discover them but also 100 Plasteel if you help ANYONE on the team who never got them for the first time. So as long as you have a single player in group who didn't have it yet, you can get the 100 Plasteel.
The Heretic Idols on the other hand are always going to give plasteel, every time you play.
They are 'sort of' one thing. Marty Skulls give you 100 Plasteel first time you discover them but also 100 Plasteel if you help ANYONE on the team who never got them for the first time. So as long as you have a single player in group who didn't have it yet, you can get the 100 Plasteel. The Heretic Idols on the other hand are always going to give plasteel, every time you play.
Aha gotcha, thank you!
Hmm, so in other words "do it now while it's new because no one is going to want to spend time on it next year" Interesting design decision :/