Possibly, you didn't question when I said the CAI was the premium designation, but considering the scandals this could be in doubt. Also, lots of people mention ACCA and CIMA being the toughest - so it's not only my, but common, albeit not universal, opinion.
I've heard the quality of ACA students debate before - You'll see from the link above showing that most ACCA students have degrees. Also, by the time ACCA students get to the final stage they are past degree level ability, yet the final level pass rates are still very low.
Also, the current CAI pass rates which you may be aware of were drastically down - how do these pass rates sit with your assertions?
While the Big 4 may want the best students, it doesn't necessarily mean that the best students want to go into Big 4 - That's CAI practice blinkered thinking ACCA, unlike CAI, does not concentrate on practice and so allows an alternative route.
I doubt you're right about mature students - from what I understand they do very well when they go back to education and presumably the same applies with accountancy.
Finally, while CAI students get better holidays, they get them in block which may not be the best, but further it's somewhat if not entirely cancelled out because Big 4 students work considerably longer hours.
Now, as I've said the perceived difficulty of the CAI is up for debate considering the new pass levels. It will be interesting indeed.
That ACCA is tougher is a personal opinion, and should be marked as such, in of course, my personal opinion