Hi all, Hoping to get some input from the forum. Background: Looking at either a Sigma 120-300mm f2.8 or 300mm f2.8 Will be purchased used. Will be mostly used to shoot motorsport, so good AF a must. I already own a sigma 70-200 f/2.8 plus 1.4TC. Now my thoughts: I have read that the AF is slower on the zoom compared with the prime, is this a very noticable difference? The prime hasnt got OS, whereas the newer versions of the zoom have - but are more expensive (I do have a monopod, but prefer to shoot handheld). The older versions of the zoom do not have OS which balances the field. What will be sharper at 300mm? Im guessing the prime? I know the main question is what focal range i will be using - the fact that i have a 70-200 and a TC negates alot of the zooms range. Using f4.0 with the TC isnt bad because motorsport isnt really shot at f2.8. I feel like i am better off finding a prime as it will be slightly cheaper overall. Thoughts? User reviews?