The 300 f4 IS, the300 f2.8 IS II is obviously faster, sharper and better with tc's(also is heavier)
But it is not 5-6 times as good..,
the 600 IS II is sharper, faster and can go to 840 and 1200mm, but it's not twice as sharp as the 300 with converter, and it cannot become f2.8(300mm) or 420mm 1.4,
What is sane?, If you don't earn a living with photography you need to look at the money you be losing when re-selling it in 5-10 years.
the interest you would have gotten from the bank , and the possibly slightly lower resale value of the lens.
I bought mine for 48 euros, price new is 68 here now, I think i can sell it for 30-40 euros in 5 years.
8 euros + the lost interest i would have gotten on the bank (interest is like 2-3% only here now), makes for a pretty ''cheap'' rent of such a lens. It is the rent I would have to pay to rent it for 4 weeks.
Alternatively you can go for something like the sigma 120-300 Sports, but i suspect that will get a price drop sooner or later.
It might hold on to it's value if sigma will not release another update within 2-3 years.
Beste Telephoto clarity for the money would probably be
300 f4 IS/400 f5.6
70-300 L IS
Then all the more expensive canon lenses (200 2, 300 2.8,400 2.8,etc etc etc)