nikon 300f4 af-s + tc14ii vs sigma 150-600 c. (1 Viewer)

i am thinking of getting myself the sigma 150-600 c lens for birding purposes in the near future.
i currently use the nikon 300f4 af-s and tc14ii with the nikon d7100/d7000.
is it really worth me upgrading for the extra focal length or should i simply stay as i am and crop images with the nikon lens?
the zoom action and os of the sigma are attractive to me but the sharpness and cropability of the nikon have served me well so far.
i would also consider the sigma s lens but i am concerned about its weight.

stop being a wimp ,i'm nearly 70 and i had hold the S all day long LOL super lens go on the thread

Nothing wrong with The Siggy C, as far as I can tell there is very little in it as far as IQ goes. The perched bird was snapped yesterday with the 'C' at 600mm and 1/400 sec hand held - I doubt if the S would have done any better.
The C is also pretty handy for BIF as the second shot shows. BTW the extra focal length will certainly give you better IQ than cropping with your current set-up.