I have a 300 2.8L w/o IS that is my primary lens. I shoot mainly sports (primarily field sports)
I prefer the non-IS version for several reasons.
Cost (+~$800) for IS, which is useless for sports.
Weight and the badly designed tripod collar. You can remove it, but if you try to rotate the lens, it is sticky and feels like there is something impeding the motion. There isn't if you pull the front of the lens up, it will rotate smoothly.
This is a problem with most of Canon's long IS lenses, it is not isolated to the 300 2.8.
I think IS is a waste if you are shooting sports, it won't help for most things, except dragging pans, but I think the cost on it is prohibitive.
I have not noticed any difference in quality shooting with the Mk1 version vs the 300 IS.
I recommend the lens geometry, it is amazing used prime or with the 1.4x TC, I don't care for it with the 2x TC (Mk1 TC) I have to test with the MK2, but the picture was soft too many times for my tastes when shooting with the 2x (f5.6 600mm) The reach is impressive for the package, but the focus was a problem.
Get the non-IS unless you need the IS for something you do.
My favorite lens by far. (till that 400mm falls into my hands)