Difference is in the quality of the optics. Better ground glass equals less distortion, more light transmission, better edge focus, etc. It will also get you better quality reticals. My "highest" end Leupolds are VXII's which are good enough for my purposes. If I were to get into longer range shooting (consistently above 300 yds) I'd go to a VXIII.
My favorite Leupold is the VXII 2-7. It is very compact and light - perfect for your 45-70. You can pick one up for under $300.