It’s actually neither. I don’t think this issue has been properly solved yet.
Though I think I’ve found a good starting point for this…
Is a good white paper on how to do this.
You’ll see equation 2 and 3 on page 2 is how you figure out the Roll/Tilt (via the Gravity vector)
The Roll can then be deducted directly from the Tilt vector.
I’ll keep you posted with progress I make with this.