Optical quality for the Canon 15x50 is pretty dang good in my opinion. Definitely better than the non-HDX 16x42 Zulu6 that I had previously. The Zulu had more CA but was still very usable and the Canon is not completely free of CA. The Canon 15x and 18x have one UD (ED) lens while the 10x has two UD lenses. Some regard the 10x with those two UD lenses to be at or near the premium level of optical quality, for whatever that is worth.
I think that my ED50 spotting scope with 16x fixed EP has better resolution than either but that might not be a fair comparison. I have not seen the new HDX though.
I have only directly compared the Canon 15x50 to the SLC 15x56 one time and it was indoors at an expo center. However, I used a textured grid pattern as an optical target to get a rough idea on resolution. The thing that was most noticeable was how much brighter the 56mm was. But with that brief viewing, using tripods, I couldn't see any extra detail with the SLC than the Canon. Obviously not a exhaustive test but I was pleasantly surprised.
Of course the Canon brings IS to the table compared to the SLC. I have used the Canon standing freehand to spot impacts at shooting matches, and just finding targets. It's awesome for that. And on a tripod it works really well too. Some people wonder why one would use a tripod with IS but they compliment each other. High wind and bumping the tripod have minimal effect on IS. So you might get away with a lighter tripod with IS.
The Zulu stabilization is even better, but the Canon works fine. I used the Zulu to watch bucks at one and two miles seated with no tripod. One mile is pushing it, but they are lighter than the Canon.
My suggestion would be to try the Canon or Zulu HDX, or both, and see what you think. To me, 15x or 16x with IS is something that I don't want to be without although I would still like to rent 15x SLC for a better comparison!