I've been investigating this since release. I'd be curious what your set-up is when mining (gear+level etc)?
From what i can see from our balancing and live rates, and even what's being reported on the wiki, Primal is a better method. Mining has this quirk that the levelling benefits can greatly affect XP output, so what you might find at the staring levels of a tier of metal is that your XP is reduced because you've yet to unlock those benefits. This is especially true if you're not using a Primal or L&D pickaxe.
I'm seeing that now between my 120-all/BiS gear on main vs. my Ironman M&S pure (don't laugh). But still i'd be curious in what your setup is, i don't want to sound like i'm brushing off the comment.