Ironically the Canon 600mm f4 IS sells for $2500 less than HoyaPentax' newly discontinued, special order only 600mm f4.
What could one do with $2500 savings?
Buy a Canon 7D & also buy 1.4x TC and still have $500 spare change.
Or add $200 more & get a 5D Mark II instead & assuming Pentax would let you special order FA 600mm f4 tax free.
If you gotta pay the state tax for the Pentax Special order then there's another $800 to $1000 to add to the cost difference between Canon and Pentax versions.
I handled a Pentax 600mm f4 FA ,when a like new used one was available 20 months ago locally, right after Hoya terminated the two local pentax reps. One Rep was called and he passed due to his newly unemployed status.
600mm FA is a crazy huge lens, 5 pounds heavier than Canon's 600mm. I passed on it a few times and bought Two new 5D and new eos glass instead. It took the shop 4 months to sell it.
That 600mm FA sure is a pretty lens though, but insanely overpriced.
Canon | Telephoto EF 600mm f/4.0L IS Image Stabilizer | 2534A002