A 45-70 when loaded full can be a bruiser over a 45 round match. A 45-90 just that much worse. Both allow some variation of powder, wad, filler , grease cookie, bullet weight, etc. I load my 45-70 trapdoor cartridges in a 45-90 length with the big 457-125 bullet so that when the action is closed the nose of the bullet is pushed into the rifling. (You don't get the camming action with the Sharps action.) It helped accuracy for the trapdoor. It used to be that brass cost was a huge factor because 45-90 brass cost triple or quadruple the cost of 45-70. Reloading dies/equipment was also expensive for the 45-90. The brass cost is not nearly as high today and relatively inexpensive dies are available.
The other factor is barrel length. In a short barrel, the difference between 45-70 and 45-90 is really no difference. The powder still has to burn before the bullet leaves the bore and in a short barrel gun, the slight extra can burn mostly in muzzle flash.
The other consideration is what you want to do with the rifle. If you will be shooting matches to 800 yds, that is different than hunting elk at 200 to 300 yds. An occasional hunting shot is much different than regular target shooting.
