Well there is a difference, although the rifles look exactly the same.
First off, the ammo is a lot more expensive. 100 brass cases cost $75 whereas I can get the same number of .45-70 cases for about $21. Here is a .45-90 and a .45-70 side-by-side.
Secondly, this baby kicks like a mule. With a 405 grain bullet, I definitely used a recoil pad, though I usually don't with the .45-70. With a 500 grain Postell bullet, I seriously consider using two recoil pads, especially since I fell and injured my shoulder earlier this year.
Jethro