I have owned the 40mm and now I have the 35mm f2IS and the later is sharper and has IS that allow you to take pictures in very low light at low speeds. It is a general purpose lens in APS-C and excellent walkaround in FF.
If you are not planning to move to FF in the near future, I recommend the 17-55 f2.8IS and the Sigma 18-35 f1.8. These two lenses are made for APS-C and offer better IQ and sharpness than many lenses made for FF.
Since you are familiar with both the 40mm and the 17-55 2.8, could you draw any comparison between those two lenses at the 40mm focal length that they both share? Is the zoom ability the primary reason to consider the 17-55, or would I also notice an increase in performance at 40mm (due to better IQ, or even just due to IS)?
Or to make things more complicated, what about comparing the 35mm prime to the 17-55 at 35mm?
I ask these questions, because I think I'm more attracted to a lens that will get me the occasional stunning photo than I am to versatility. If the 17-55 delivers both stunning photos and versatility, great, but otherwise I think I'd still be leaning towards the Canon or Sigma 35mm.
I greatly appreciate everyone's feedback. Please know that I take all the replies seriously, and use them as a launching point to watch youtube videos and generally learn more about all the lenses you all are recommending.