As I see it, the key difference is timing. Royal Carriage lets you repeat an Action's benefit at once, but you have to have it in your hand when you play the action
I believe you mean on your Tavern mat?Well, yes. But if you play it one turn and call it the next then you're trading off increased flexibility against the Duration-like limitation of only getting to play it every other turn?
Scepter is way weaker than Royal Carriage, but there's some cool stuff it can do that RC can't, and in the occasional game where it's useful or important it can really make the difference. There's a huge combo with Sculptor and any cost reduction for those who want to find it, but even just using it to play more Monuments or Wild Hunts or Bridges can be devastating if you're the person who sees it first.