Upload & Sell: Off
I am not looking to make any cash from my tele work, it is purely for enjoyment and for me using a zoom makes it a lot more fun.. I plan on taking in a lot of motorsport next year and the zoom for me will make it less hassle and will save me having missed photos from not being able to zoom out.
I had the Canon 300mm f4 before and really did not find it as much fun, ended up carrying 2 bodies and missing shots.
For me I would much rather have the Sigma but if you need every ounce of IQ and have time to change lenses/TC's oh.. and an extra £2000ish then the Canon is great.