Which lens for motorsport? Sigma EX f2.8 70-200mm or EX f4 100-300mm

Hi everyone

I want a good lens for motorsport use, I've narrowed it down to 2 lenses. The Sigma EX f2.8 70-200mm and EX f4 100-300mm. Both are within my budget.
I can't always get access to media section so will have to shoot from grandstand/spectators area.

Will 70-200mm be long enough or am I better off with 100-300mm?
The 100-300mm does have a few stops lower so therefore it maybe difficult to shoot in cloudy days.

Which one should I buy?

Depends how much money you have and what sort of motorsport you want to photograph - some happens further away from you than others.

I find that for circuit racing, its normally between 200mm and 300mm.

So either the 70-200 f2.8 with a 1.4TC (losing one stop) or the 100-300 f4, the benefit of the TC is that you can take it off again if needed.

If it gets cloudy, you have the option of increasing the ISO a smidge.

i have the 100-300 and it's simply awesome. plenty sharp wide-open, outstanding build, and the hsm has never let me down, even shooting in ai servo on a 1d- arguably the most demanding of af speed.
i plan on replacing it soon with a canon 70-200 f/2.8 IS, but that has more to do with the IS than anything else.

After some researching. I've decided to get the 100-300mm.
I'm now £517 lighter on my bank balance

I've just paid for a used one. I'm £ 390 poorer, but hey, its only money.

Get yourself a 1.4xTC and a strong monopod and you should be set up quite nicely

Already have a Manfrotto 679B, it's max load is 10kg which should be strong enough for the heavy weight lens.
Can't wait for the lens now, it should arrive in time for Renault World Series at Donington
1.4xTC will come soon after pay day