I've been using a Tamron 17-50/2.8 as my walk-around lens for years (with my 7D) and have loved the IQ from this lens, but have always disliked the loud and relatively slow AF. Since I'm adding a 7D Mark II to my collection, I figured maybe that it is time to get a new walk-around lens. I'm trying to decide between the Canon 16-35/4 or the much-less-expensive 17-55/2.8. One immediate advantage I see to the 16-35 is the weather/dust sealing. Is the IQ difference very great? Any thoughts on this decision will be appreciated. I'm not planning on going FF so the EF-S designation of the 17-55 doesn't bother me.