I am tossing up whether to purchase the Canon EF 70-200mm f2.8L IS lens or go for the 100-400mm.

I mostly like to shoot sporting events (particulalrly equine sports) and ocassionly birds, wildlife etc. I was thinking that the 70 - 200 might be more practical for everyday stuff and a perhaps use a teleconvertor for the ocassional bird shot but I did hear somewhere that if you're considering a teleconvertor then you should go for the bigger glass to begin with.

At the moment - I am only using an entry level EOS 1000D which I but I plan on upgrading to the 7D.