I like the songs equally much so here's my reason for wanting BTL to be booted: FF is simply more essential to the album than BTL is. Think of the album without FF, now that would be horrible (I mean, imagine using TSPCO as the final act). But if you remove BTL, it wouldn't make such a big difference, at least not musically for the album as a whole.
Ooh, opposite here!
Finally Free is the epilogue, I've always thought. Nicholas has been on a journey, he's learned a lesson, he's jubilant... but then, once the credits have rolled and the lights have gone up,
then comes the twist.
The Spirit Carries On is one of the band's signature closers. It's the climax, it's the end of the story. It's Frodo getting his finger bitten off. Finally Free is some hobbits fucking around on a boat. The extra bit. Nice to have, adds a lot of context and wraps up the last questions about what happens to all these characters, (in this case, that they all die horribly,) but the last narrative beat was twelve minutes ago.