From the couple reviews I've read, the newer f/4 IS 16-35 is apparently, one of Canon's very finest wide zooms, with very good center and edge performance and super-low chromatic aberration, AND a really good IS system that works great. Like a lot of people, I see IS/OS/VC/VR as a tool to be used for several things: first off, for the best possible panning shots; for shooting well stopped down (f/11 to f/22) to pull deeeeeep depth of field in places where tripods are either not allowed or are impractical for whatever reason; for use in the wind, or when shooting out of breath, or aboard boats, trains, ferry boats, or other unstable shooting platforms, and for using with slow shutter speeds when dragging the shutter with flash, so for shooting f/8 or f/11 at 1/4 to 1/8 second at boosted ISO values, stuff like that. SO to me, the IMage Stabilizer feature is actually MORE-important than a marginally usable f/2.8 aperture.
Tamron 15-30? I have not seen any MTF charts for it, but in that range, a lens needs to be at least decent optically, to gain any real traction as a third-party lens. Without Canon's L-series red stripe or Nikon's gold ring, an independent lens lacks the automatic, built-in pedigree of a Canon or Nikon top-shelf model, so, one would expect the Tamron has been designed and built to be GOOD; and it needs to be because of what the camera makers offer in that slot.