300 mm or 400 MM "Which one to get"

[Sifor]

I'm a Nikon shooter but in a similar dilemma to you regarding super telephoto gear.

May I suggest the Sigma 120-300 2.8 which retails at the moment for $2k (due to a new version on the horizon). I suggest you read some of the very favourable reviews (on balance & cost) of it. In a nutshell:

Pros:
Centre sharpness @ 300mm 2.8 equal to the name brand primes.
Zoom
Handles the 1.4 & 2x TCs well
Cost ($2k after CR Kennedy price match)
OS is very very good (e.g. sharp shot 1/100th at 450mm...)

Cons:
Corner sharpness not that crash hot.
Heavy (3kg)
Apparently some issues with the OS failing (hence the new revision being released in 2013).
AF not in the same league as the name brand primes.
No focus limiter (included in the new version)

Worth a thought I'd think, 2.8 glass is always great.. especially when you can add some 1.4 (168mm-420mm f4) or 2.0 (240-600 f5.6) TCs to cover the entire 120-600mm range with a single lens for $2k... yes it won't beat a Nikon or Canon 300 2.8, but for less than 3 times the cost, it certainly isn't 3 times worse