Looking at the 500 mkii with the 1.4x or the 300 mkii with both mkiii converters.
I like the versatility of the 300 as a 300, 420, or 600 but prefer the reach of the 500 (or paired 700).
I shot an mki 500 in Africa for 4 days and loved it. I can't imagine I would dislike the 500 mkii but I'm concerned the 300 f2.8 would be a nice option.
I have a 70d, 6d and 100-400 mki and tamron 70-200 f2.8 already. I primarily shoot wildlife. The 100-400 never is enough for birds which is why I was looking at the 500
I was in the exact position about 9 months ago.... long story short, i went for 500L, and I DO NOT REGRET IT! If you want length, get that... Sure, i wish i had the 300 as well, but I would not exchange!
If you're unsure... rent both and see.
Either way, enjoy!