The whole dust issue in the 100-400mm is an urban legend. I use mine on polo fields with horses flying by all the time and I have never, ever had a problem with dust. EVER. Somewhere, somebody decided that the push-pull system just HAD to attract dust, the idea caught on and it's attained a life of it's own. It's crap. It's not a problem. It never was a problem.
Now image quality... that's hit or miss. And the autofocus is slow. I don't love this lens, but it's serviceable.
ditto for the dust on the 28-300 never had a problem with it, once ou get the hang of the push pull zoom its very fast I found the AF speed on the 28-300 very quick, maybe not as quick as the 70-200 f2.8 but not too far off the mark, but As far as I know the 28-300 is rated as weather sealed where as the 100-400 is not "weather sealed"