I am glad I found this thread as I had become frustrated of going round and round the 'which lens to choose' routine which is usually compromised by either not having enough money to buy what you really desire (but might not need) against not quite knowing the distance between camera and subject for which the lens will be needed most. First question to 'bdigital' though, if still on the net, which lens did you go for? My money would have been on your choice being the 100-400?
I am going through the same quandary with the same amount of money but I could push it a bit further. I currently have a a Canon 7D with a 70-200mm F4 IS etc and I use a 1.4 extender at the expense of a stop. I use this combination as a spectator at motorsports events: BSB, Thundersport and MX and the like. For some circuits such as Cadwell this is fine but at others it is underpowered yet the reach of the combination is 448mm. When I get it right my shots are pretty good, sometimes very good, but the wastage hints at poor technique or expecting too much of the focusing system which is trying to find definition in a very short space of time. So my mind turned to looking for a bit more reach and putting the 70-200 on my 30D (I know it's almost prehistoric now but still works well).
The lens that leapt out first was the Canon 400mm (640mm )which takes the extender but at the expense of the autofocus and hand-holding at that range as no IS; my eyesight isn't what it was so I went off the idea but it fits the price and quality. I then thought the 'white' 70-300 F4/F5.6' which has good reviews but that appears not to take, or be compatible with, the extenders (?); also a lens mount is not included - so a great all-round lens but not top of the leader board for me. The 100-400 is also popular and I know a few photographers that use them (again no mount included) but I am not sure about the bellows at MX events and the advice from you all that you will spend your time at the 400-end rings true. I suppose a makeshift or cover could sort that out. So I came down to the Canon 300mm F4 (480mm with camera crop) which fits the bill financially along with the quality - it is much lighter as well and I have also looked at the Sigma 120-300 F2.8 (better focussing opprortunities for sure but another £1K or so) but that weighs in at 3Kg plus, which is a lot to carry about for a day, perhaps with the second camera and lens, waterproofs and a half-eaten bacon buttie! I know some of you will probably be saying: 'what do you want, mate, the shot or a compromise?' The best compromise I suppose.
So much as this thread suggested, I think the 300mm will be the one and I need to check one out, but I hope my thoughts might help build on those I have have already found so helpful here. The big question though is: if we, as spectators all have to stand at the same corners with the same cameras and lenses then getting 'different' shots is very difficult. I used to ride MX and enduro (a very long time ago) and love the day out whether I get good shots or not, but access to events is very important as well when thinking about lens choice. I no longer go to some British MX Championship rounds as so much of the course is roped off to the public (I can understand why given the costs of events these days) that the photo opportunities from the crowd line don't justify the effort and cost, from my point-of-view. Cadders Hill at Lyng in Norfolk is an exception though, as is Hawkstone Park - I have ridden them both and still admire how quick the riders go round.
Again, thanks for all the contributions above which have helped my thinking greatly. By the way, for anyone who has stayed with this message and knows Brands Hatch (I haven't been before) I hope to go in August to see the F3 meeting. Will my current set up 70-200 and extender be man enough for that circuit?
All the best