Agree with you Harry, not really the mission set, but much more to with what you can afford. Cat and Trap carriers are much more expensive to build, operate and maintain, but most will admit that the air-wing is much more capable. Very few navies have been able to afford true cat and trap carriers.
As for the size of the air-wing, "only" 48 F-18's is still quite an air-wing- larger than quite a few nations, and very few hangar queens. Capability of the air-wing must also be considered, not just the numbers, although numbers are a quality all their own, and QE will be much more capable than the INVICIBLE class.
A quality air-wing that can provide fighter, strike, AEW, ASW, anti-surface, refueling, COD, jamming, etc. with aircraft with adequate range and payload (CATOBAR) is tremendously valuable (and very expensive). While the F-35B will be much more capable than the Sea Harrier/Harrier, there is obviously a tradeoff with going to STOVL carriers and aircraft, with the inability to have fixed wing AEW as arguably the biggest loss.