From my own experiance you get more skills on "condition" than on "quantity" (If only slightly) so it would be in keeping with this that the BP QR went up faster too. However, i suspect the vastly reduced returns on "condition" would result in a higher cost to get to 100qr, but you would do it faster.
This would be even more true if using a skilling BP like dumpers, as they have fixed "condition" anyway. As I see it the only real use of the condition bar is to get globals. In most cases, it's probably cheaper to go on quantity as it can either be repaired in the TT, of have the TT boosted with residue.
On the other hand if skilling on non-stackables, like settler shins, you are probably better off on "quantity" otherwise you end up with 100's in your inventory, and it takes ages to TT them.
Opinions only, as I have no figures to back them up.