Sara, I did both rim to rim AND Whitney as day hikes.

As you read, I found the rim to rim hike a real test -- and for me, harder than Whitney.

The reality regarding which hike is harder may not match my observation. (And I'm not implying that the reality is that Whitney is harder. That reality is, to my guess, unknown. I.e., I don't know of any concensus opinion regarding which is the harder. I don't know of any survey/poll conducted amongst many joint rim to rim/Whitney day hike veterans.)

Bear in mind that my opinion may have been influenced by both my older age in doing rim to rim (66 vs. 59) as well as arguably poor hike management doing rim to rim (not enough or/and long "breaks" during my descent before hiking uphill, as well as not enough fuel or liquid intake before the climb up).

Welcome to this forum! Please post again or PM me if you have any further questions.