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I don't use Canon myself, so I don't know these specific lenses, but in general terms, I'd say your choices are:
If it's a straight choice, & you can afford either, seems like a no-brainer. Go for the fast lens. You'll only wish you had at some time in the future.
If you have a certain amount to spend, & want another lens or accessory with what's left, go for the f4, as the extra stop may prove less useful than the other item.
You've not really been specific enough about why there's a choice. Is it budget, thinking you may not need the extra stop/expense, thinking of buying something else at the same time?
The way I see it is like this:
Both lenses are going to be sharp and optically sound choices, the f4 version is even generally rated at being a hair sharper than the f2.8 at f4 - however we are splitting hairs here as both are very high grade to start with. So I would not worry about overall optical quality from either.
Secondly you have aperture - the f2.8 offers you wider max aperture and the option of more light to be used in dimmer conditions whilst also giving you the creative option of being able to shoot at f2.8. Further more if you consider using a 1.4 teleconverter the f2.8 again gives you more aperture to work with in comparison to the f4.
Thirdly you have size and weight, the f2.8 is larger and noticably heavier than the f4. This is sometimes the major point for most people and the first time you see/use an f2.8 it feels massive and overweight. However (and this comes from someone who is not big nor muscly) the f2.8 version is certainly within most peoples capabilities to grow used to and given a month of so of good use and you can easily use it.
Fourthly there is IS - something that the f4 version gives you that the f2.8 lacks. Remember though that this is only going to counter your handshake motions. For something like motorsport this might not be an issue as you push for faster shutter speeds.
There is a 70-200mm f2.8 IS L however its not out of production with second hand going for about its original new retail cost and its retail cost being totally out of control (most places just have no stock at all). The M2 of this lens is a massive jump in price from the others you're considering so whilst I'll throw it into the pot to consider I suspect its far more in cost that you are willing to invest.
I chose the f/4 IS version. It is half the weight of the f/2.8 versions, which are really heavy lenses - about the same as the very substantial 100-400L. Get to try before you buy 
F/2.8 is obvioulsy nice to have, but unless you use it on a fairly regular basis you are carrying it about for no benefit. Also, looking at the two lenses you have selected, you will miss out on the finest incarnation of Canon's four-stops IS system, which is arguably more useful than an extra stop of aperture. Plus the f/4 lens is fractionally sharper, it has weather sealing and a cirular diaphragm for nicer out-of-focus background effects.
I've taken many sports and wildlife shots with my f/4 lens and always been thankful that it's so light in comparison to the f/2.8 and never really suffered by my choice, especially with the 1.4x extender.
However, remember that the f/2.8 can be used with the 2x extender as well as the 1.4x, whilst retaining autofocus, a plus point that can be very useful.
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