Interesting question re: difficulty of rim to rim vs. Mt. Whitney main trail in a day.
It's been discussed before, and I can't remember what the results were (and am too lazy to research it) -- but I tend to think the answers skewed toward Mt. Whitney.
I'll have my own personal feeling about this after I do my rim to rim hike on October 3.
At face value . . . 22 miles round trip for Mt. Whitney vs. 24 for rim to rim (North Kaibab & Bright Angel trails). 6,100 feet elevation gain for Mt. Whitney vs. about 4,500 (after going down about 6,000) for rim to rim. Lower elevations for rim to rim (highest about 8,100 vs. the 14,500 for Whitney). But going UP at the END instead of the beginning for rim to rim (I'm quite interested in what THAT will be like -- it's 15 miles and about 6,000 feet down before then going another 10 miles up some 4,500 feet).
(All the above is predicated on a rim to rim hike in reasonable temperatures -- May or late September/early October. Throw in hellish heat at other times for the rim to rim and the answer might well be rim to rim.)
Is my memory correct that the predominant viewpoint is that Whitney is a little harder? Folks?