I have a Canon 70-200mm F4 L and a Canon Extender EF 2X (series 1). My main focus is shooting birds from a distance. I would like to get the 2.8 version of that lens but can't afford it right now. I have also read that the 2.8 version might not improve my picture quality from the F4 I am using as the optics are supposed to be very good on my current lens. As an alternative I was considering getting the Sigma 70-200mm f/2.8 EX DG APO.
Do I stay with what I have or get the Sigma?