Andrew
Your reason for wanting the Canon is very similar to mine. I was a Wild Fowl reserve and tried to photograph a group of five swans coming into land. Great formation, I was on a bank at about the same height as them . Their approach was at a slight angle, and the Sigma just couldn't lock onto them whilst they flew. I've not had any such trouble with the Canon locking onto moving subjects.
Yes the push pull construction does take a little getting used to but it does have an advantage. As you extend the lens your arm goes out with it to improve the support.
As to the fear that the lens pumps dust around inside the camera, I had to clean my 5D sensor after using it for 3 months ( a very small spot top RH corner) and nothing for 6 months. The 20D before that got one cleaning in about 12 months and nothing in over a year. I'm sure good housekeeping helps.
Focus is coming up in a couple of months, if you can wait 'till then Canon will be there so you can try the lens for yourself. Better still Warehouse Express have just opened a shop in Norwich. They claim you can "See it, Touch it Test it"