I don’t know if this is satisfactory, because it’s just a personal sense. I’m fairly certain that there is no serious scholarship seeking to simply quantify the prevalence of diatonic harmonies, although I could be wrong. I’m honestly not sure that it’s even feasible. I have read many textbooks and scholarly articles, and have studied a metric crap-ton of common-practice music, but it’s still just a gut sense. Anyway:
I suspect first of all that any diatonic variety of ii is more common than any variety of diatonic vii°. ii is the most common and powerful diatonic pre-dominant function harmony, while vii is the less common and less powerful dominant function harmony. I can think of very few phrases of music that don’t have ii in them, but I can think of plenty of entire sections of music that never see vii at all.
On top of that, after V7, ii7 and iiø7 are the most common chords to have sevenths added to them. Admittedly, ii chords place a distant second place, but I’m all but certain that beat out vii handily. The most common usage of vii I’ve seen (outside of modulatory passages of course) are vii°6, and that only rarely has a seventh added to it.
Finally—on top of those two limiting factors—we have the fact that even when composers have the opportunity to use viiø7, they are a pretty likely to go ahead and fully diminish it (to the point that vii°7 in Major is so common that it’s barely even considered to be chromaticism), whereas I’ve never seen that done with ii without it being an applied dominant of the relative major.
So: 1) pretty sure ii is already more common than vii, 2) pretty sure ii chords are more likely to have sevenths added to them and, 3) even when vii does have the seventh, there’s a whole extra option that throw the stats off even more.
I’d be very surprised to be found wrong on this, but I’d love to see the study if someone can figure out a way to make it viable.