Nothing wrong with the MkI. I had one from new, and never felt the need to upgrade it to another 300mm.
1-stop of additional VR, and a M/A switch as well as an A/M. But as Stewart says all the main things that affect your images are identical.
Now if there was a MkIII and it used lighter optics and casing to match the Canon MkII then I think my recommendation would change..... if I had to buy one now I'd get a good condition used MkI.