Ok, well not really a vs sorta thing, but...those who have the carbon fiber...
does the relief on the board feel good/natural? I've had very limited time on one. I don't doubt the stability at all. More just the feel and playability since I've never owned a guitar with something like that.
I've had two; my orignal Tonare Grand Custom had a carbon fiber rod, and my current Private Stock acoustic has one because I liked it so much on the first guitar.
It feels absolutely great. Especially on an acoustic where action is typically different from an electric guitar. No matter what's happening in terms of temperature and humidity, the guitar just plays spot on like the day it arrived. And we had a horrible, dry, cold winter last year in Michigan. The stability in the neck is
incredible, and that helps playability more than you might imagine.
But I also want to discuss what it does for tone. I think many would agree that putting a steel block and rod inside a neck will affect the tone. No doubt carbon will, too. But I go back to the days when my high end Martins were made without truss rods, and my personal belief is that guitars without metal truss rods sounded a bit different from guitars that have them, something I attribute to the woods and construction being even more crucial to the sound of an acoustic than an electric.
The carbon fiber rods are embedded into the wood. They have lower mass. I think there is just a touch more subtlety in the way the guitar sounds on account of the lack of metal in the neck. This is hard to put into words, and I might indeed be crazy. But I do hear a sound I like better.
Anyway, so what if I'm crazy?
Paul Smith thinks they sound better than steel rods, too. Not saying my ears are as good as his, but I hear the difference.