And argument can be made that the Eagles were not exactly rock 'n roll. Be that as it may, I always liked Frey's voice over Henley's. Though Henley sang at least three absolute classic tunes (Hotel California, The Long Run, Desperado).
It might be his lower voice, but Frey kills it, IMO, on Lyin' Eyes, Take it Easy, Peaceful Easy Feeling, and took over for Randy Miesner on Take it to the Limit.
Other considerations between the two include solo work. Frey was more rock (Smugglers' Blues, You Belong to the City) and Henley morphed into Adult contemporary (Dirty Laundry, End of the Innocence, Last Worthless Evening, Heart of the Matter. Though Axl sings back up on I will not Go Quietly).
As long as I'm on the subject of bands with more than one singer, Roth in his prime was a better frontman, but Hagar destroys him in terms of musicianship and vocal range.
Also, Bradley Delp may have been the primary singer in the band, Boston, but make no mistake that Tom Shultz is/was the unquestioned leader.
Finally, '72-'76 era Steven Tyler for the win.