Upload & Sell: On
I (still) have both, they are same focal length but very different lenses. As a general purpose super tele, the 300 f4 IS is a great lens, perfect for a lot of applications from mammals to butterflies and everything in between. With its light weight and build it in hood, it is very convenient as a hiking lens. Takes 1.4x well to have a 420mm f5.6 IS lens.
The 300 f2.8 IS V1 is the cheapest of the big boy super teles you can own that has IS. Takes 2.0x III remarkably well and AF is dependable even for many BIF applications, at least with 5DIII. Very versatile both for FL and F Stop. I used my 300 f4 maybe 3 times in the past year after getting the 300 2.8.
Lets put it this way, if I didnt have the 500 and I could only have 1 super tele, it would be the 300 f2.8
Also, canon refurbished in US had the newer version in stock for ~6k, you may want to look into that as an option.