I own the Sigma and extensively used a friend's 18-135. The latter is fine by itself, with a flexible focal length, WR and SDM. However, even without pixel peeping, the Sigma is a better lens optically. For me the difference is obvious. It's faster, quite sharper in the corners, a bit sharper in the center, with a bit more contrast (that last item is subjective).
I also do a lot of outdoors activities. My solution is to use the 18-55 WR when conditions are more difficult, bringing along the 100 macro WR if needed (and now I also have the bulkier 60-250
) It would be an economical (and even more compact) solution for you, allowing you to keep the Sigma.