Imagine if scientists addressing the same issue from 2 different perspectives weren't allowed to publish seperate articles, but rather could only publish them in the forms of commentaries on the previous ones...
Changes in "how it gets done" is novel work and worth reflection.
To OP: I think you should just look at peaks, and therefore: Roger>>>>>>>>>>>>>>>>>Rafa.
But he's better because he's a better player, not because of mental strength (if that's what you care about). Looking at mental strength is MUCH MUCH harder to do scientifically, near impossible IMO. Far too complicated, far too many variables.
Edit: BTW, if you are interested in mental strength, there is a decent amount of literature for other sports (especially baseball). Worth a look at.
They can actually build from the particular commentaries and bounce ideas of each other, that is if they are trying to prove the same thing.
You can't measure mental strength, as you said too many variables, simple case Nadal on clay and Nadal on fast hardcourts. It's the same guy, but one is a comfort zone and the other isn't.
Nadal has never played a GS final at Wimbledon and lacked balls, granted he was not likely to win on grass unlike Federer did when he played the 2006 RG final. If that example is used Nadal fared better in the unfavourable environment than Federer has.
Then look at where Federer has won his events, he has done it in more places and variable surfaces, in other words, it just depends on what has greater weight to the individual judging.
There is only so much Fed and Nadal speak that I can put up with.