There are allot of great points brought up. I was also wondering about a 7D as a possibility to give me the added reach. While I'm not rich affording both in the future is a possibility in the future. For now I need to pick one. The Frugal side of me says 300mm is the way to go. But considering the 300 price is halfway to the 600 it's one of those things. No one else can make the call except me.
To answer the folks that said I didn't provide enough info.. Truth is I would shoot what ever I can with which ever lens I had. If I had the 600 I would probably shoot more wildlife. Presently I do allot of autosports. so I think the 300 would be a great lens for that purpose. But I also shoot Macro.. So go figure... LOL
Be careful to consider if the 7D will deliver high enough image quality for you, particularly at higher iso's. Coming from a 5D MkIII you may be disappointed (I just got a 5D Mark III just for this reason). My 7D is still here, but I think I may sell it if I truly find I no longer need it.
5D3, 5D2, Sony α6000, G16 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 2.0x III, 70-300L | E-mount: SY12 f/2, 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses