I vote for the Canon 100-400L. The push-pull zoom may not be the same as other 'twisty-zooms', but it is faster and more intuitive. As for dust getting inside, FOOEEY! I have 30 year old 'trombone' zooms that are as clean on the inside as the 100-400L I just bought. The zoom mechanism has nothing to do with internal contamination. A lens that displaces 100cc of air when going from max to min zoom will necessarily inhale 100cc of air when going from min to max. Trombones and twisties both move air when zoomed. The mechanism for moving the zoom is irrelevant.