I would recommend the 6D over any crop camera for all pursuits except for tiny songbirds.
As an owner of numerous Canon crop cameras the last decade, the IQ IMHO lost something with the start of the 7D. The images just look "rough" compared to Canon's FF offerings. I have also experience increased focus inconsistency with the 1.6x high megapixel sensors.
The 300 prime is amazing on FF. Also, IMHO, using a 1.4x on 50 and 85mm primes is a waste of time, just crop. The 1.4 is brilliant on the 300/F4 on FF, though.
Wait, you can use 1.4X extenders on the 50mm and 85mm? What's yoursecret?