My two main uses for this lens will be sideline football and birds/wildlife. From experience with others I know, a 500 is too long for football. I can believe this as I found that I rarely used my prior 100-400 at 400 on the sidelines. Also, that extra stop of light is critical to me for night games. So, a 500 would limit its use to just birds/wildlife for now. I'm thinking about buying the 300 and a couple years down the road adding a 600. Extenders will give me reach on the 5d, and I an gain even more reach with my 7d II for birds. I haven't shot anything yet with the 300 and 7d II.
I think you are travelling down the same road as I did. The 300 F2.8 L IS is a stunning lens (even my Mk1) but where you need reach a longer lens is better. I went for the Canon 800 F5.6 L IS and am absolutely loving it! When funds allow, also give the 1DX a try out - but not until you have the funds as you will want it!