Used the 70-200mm f2.8 MKI (the MKII much improved TC performance) with great success for the Czech TT in Horice when a contingent of Irish riders paid a visit. Never had the opportunity to go to Ireland to see the road racing, but its on my list of things to do. To be honest, the 300mm f4 had an issue and needed a service so I had no choice but to use the 70-200mm. If I had both lenses available, I would have taken both. For rallying, you'll definitely need the 70-200mm because of the differing light conditions. Both lenses are great, I've used the 300mm f4 a lot more for Motorsport than the 70-200mm, because it gives you that extra reach with the 1.4x TC and as andyred mentioned no real loss in performance or IQs. Depends what your finances are like, if you got the money I would look at buy both used, but if you had to choose 1, the zoom might give you more flexibility for the rallying and road racing.