In 5e, there's a reason to have a mix of things.
While there's lesser versions of the same idea - Advantage/Disadvantage is important in this consideration. If everything is "player roll vs NPC/monster defense" then the players always have a chance to apply Advantage, from Inspiration, or from whatever other sources they can cook up. And generally, the PC's proficiency modifier is involved.
When it is "NPC roll vs spell save" or the like, the PC's proficiency modifier isn't involved, and they are less likely to otherwise impact the roll in the moment.
So, the math is different in the two cases, leading to the players having to think a bit more about what their best strategies are.
While there's lesser versions of the same idea - Advantage/Disadvantage is important in this consideration. If everything is "player roll vs NPC/monster defense" then the players always have a chance to apply Advantage, from Inspiration, or from whatever other sources they can cook up. And generally, the PC's proficiency modifier is involved.
When it is "NPC roll vs spell save" or the like, the PC's proficiency modifier isn't involved, and they are less likely to otherwise impact the roll in the moment.
So, the math is different in the two cases, leading to the players having to think a bit more about what their best strategies are.
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