With all the new axes out it might be different, but I've always gotten higher numbers with Primal Rend, depending on what I'm fighting.
Some mobs like the pots in Sky, Tonberries, Aerns, etc, have hard magic resist, and I generally do more to them with Rampage. For everything else, there's maste.. I mean Primal Rend.
With a Woody/Maneater, my rampage as bst/nin would do anywhere from 400-700 on a normal mob with ok evasion/defense. On that same mob, or any with higher evasion/defense however, I will get a steady 860-950 Primal Rend, depending on their magic resist, and other factors(Shell, etc). The con to using PR is, like how you can miss with Rampage, you can be resisted, and it's easier to raise accuracy than it is to raise magic accuracy with bst. A lot easier. Fortunately, you won't be resisted very often unless you're trying to fight something endgame related, and not even a bst enthusiast would bring it to a byakko fight when they have Sam available.
With these new axes though(I'm looking at you, Glyph axe) the damage on them might be high enough to press PR a bit, but there are some positive points which really make PR shine:
Light-Element
Not reduced by slashing resistant mobs(ghosts, slimes, etc)
High CHR modifier as primary stat
Can open/close both light/dark
Bypasses invincible/Perfect Dodge
Very consistent damage
Will. Not. Miss. Ever.