To be honest, before seeing this article, I'd never really thought of "Freebird" and "Stairway to Heaven" in the same terms. They are both epic rock songs and have a good bit in common. Both start out slow and build into frenzied guitar solos featuring amazing guitar work from their respective musicians. That being said, I feel as though "Freebird" is the superior rock song.
"Freebird" is a bit more accessible lyrically. It's easy to understand what Van Zant is singing about, but it's simplicity doesn't detract from the overall quality of the song. Robert Plant's lyrics in "Stairway to Heaven" are somewhat cryptic. It leaves the listener wondering exactly it is that he's singing about.
Both songs start with simple guitar passages, but for me, the slide guitar accompaniment in "Freebird" really sets it apart from "Stairway to Heaven." Gary Rossington does a tremendous job of imitating a chattering bird during the intro.
The outro solos are where both songs really shine, however. While both solos are tremendous, "Freebird" really wins this one through sheer man-power. Lynyrd Skynyrd typically has three guitarists on stage as opposed to Led Zeppelin's lone guitarist, Jimmy Page. Skynyrd does a an excellent job of weaving the solos of the individual guitarists into one dynamic composition. In some parts, the solos are independent of one another, but they always seem to intertwine and complement the work of the other guitarists. "Stairway to Heaven" features a bit more concise solo, and it's no less awe-inspiring, but it's just over powered by "Freebird."
Although both songs are in heavy rotation on every classic rock station, "Freebird" enjoys a bit more exposure in modern media than "Stairway to Heaven." Lynyrd Skynyrd still tours (with some different members, obviously) and they end every show with "Freebird." On the other hand, Led Zeppelin hasn't toured in 20+ years. The opportunity isn't there for new fans to see them live. Lynyrd Skynyrd also has their songs featured in games like Rock Band and Guitar Hero. This brings the song to an audience that might never hear it otherwise.
Both songs are great rock songs, but I just feel like "Freebird" is more of a transcendent anthem. "Stairway to Heaven," while far more than just another classic rock song, just doesn't have that same kind of timelessness that "Freebird" does. I have no doubt that both songs will be remembered for many decades to come, but I believe "Freebird" will have the greater legacy.