Aiming: Occurrence of over/under aim, number of shots fired versus head shots, that sort of thing. Creating a statistic record of the average 'rating' a group of individuals whom have played both would assign to each. The last part is hypothetical, but being able to observe our own occurrences of aiming 'error' and our own experiences with tracked stats between the two games certainly isn't.
How many more instances of death due to being locked in to melee happened in uncharted 3 versus 2? Probably more.
Actually this is a point that should be made, the reason that Uncharted 3 is harder than the previous games is due to multiplayer. Naughty Dog specifically said that one of the lessons they learned with uncharted 2 was not to create a health system that was different between the single and multi. Well, they balanced the game around multiplayer, and as a result Drake takes more damage than he use to in single player. It also doesn't help that many encounters in 3 are in the same vein as the Chateau encounter with Sully from uncharted 1, namely "snipers, rockets, shotguns all at once'.
That doesn't "measure" whether aiming is "worse," just whether it is less accurate. If they had made it significantly more accurate than U2, in my opinion that would be "worse," as it would make the game far too easy. Less-accurate aiming would make the game more difficult, but that doesn't make it worse.
And it's not the only thing to take into account for a game's difficulty because yo, I think U2 is more difficult than U3. Like... it's not even close.
It's also way better.