Regarding weather sealing...do any 35mm primes offer it? I didn't think they did. The Canon 35mm f/1.4L doesn't appear to offer weather sealing.
I am heavily considering buying a full frame body soon, and am torn between buying a zoom and buying this Sigma 35mm prime. The IQ of this Sigma, along with its aperture speed...absolutely blow away all zooms other than perhaps the Canon 24-70 f/2.8 version 2...where the resolution might either be equal, or not much different (at various apertures from f/2.8 and smaller).
Ideally I wanted to wait until a true deal was offered with a body plus 24-105 kit, because that lens is a world beater, given its IQ at its price. But obviously it would be very inferior to the Sigma 35mm prime (at that focal length), for not much less money (especially absent a true deal on the kit). The advantage is the zoom ability, obviously...and to a lesser extent, the IS.
I don't recall the 24-105 I used, having weather sealing of its own, but I could be wrong. Certainly some dust will enter the internal elements via the telescoping zoom...but I think it might have had a small amount of foam or felt at that joint as well (might be wrong on that too). However, even if it did, it wouldn't be 100% effective.
Regarding resale value, I happen to think this particular Sigma lens will hold its value very well for many years to come (especially considering the price when new, is a true value, as opposed to Canon's aging offering.) It looks like I'm not alone.
I have owned, and currently own, several Sigma lenses of various types, and have only had autofocus issues with one of them. I've had no other issues at all. I also own, and have rented, top quality L glass, including supertelephotos. All of the Sigma lenses I've owned or used, have been either the equal of comparable Canon lenses (including L lenses), or else offer 97% of the image quality for anywhere from 75%, down to 50% of the price. Snobbishness towards the Sigma brand is becoming less relevant all the time nowadays. The redheaded stepchild of Japanese camera and lens companies, is rapidly rising to prominence.
There can surely be no serious doubt, that this Sigma 35mm f/1.4, is superior to Canon's current 35mm f/1.4L. Perhaps Canon will be forced to finally bring their updated version to market. Or perhaps not, they seem to be more interested in making cheaper, smaller, less useful rebel-like bodies for the masses.