I think Claire A is the easiest of all of the scenarios. B scenarios are automatically harder due to the increased presence of Mr. X and I feel like there are additional enemies in the B scenario while actually being slightly shorter than the A scenario.
Now, when we do the Claire VS Leon A scanerios, here is why I think Claire has an easier time:
1) Mr. X is murdered in her scenario. And even though Leon only has one more chase sequence, this is non-existent in Claire's scenario.
2) The Birkin end game fight for Claire is way easier than the Mr. X final fight in Leon's story. Claire starts off with a mega strong machine gun and has so much room to roam and get away from Birkin's attacks.
3) Claire has avoidable Lickers in the RPD Underground while Leon gets assaulted by a thousand dogs. The Lickers are way scarier, but are inevitably easier to deal with.
It is an open debate as to who has the better loadout, and while I lean towards Leon having a better overall set, it is pretty much even.