Just in case anyone else reads this thread, the Tokina 120-400 generally gets terrible reviews. You would want to be really, really confident that it was going to do what you wanted before you spent the money - and I suspect that anyone who does do their research winds up buying something else, because you never see this lens around anywhere.

If you are scratching for the dough to buy a 400mm-class lens and can't afford a 100-400, look at the superb Canon 400/5.6 (but beware - it has no IS) or else the well-regarded Sigma 150-500 OIS.