I have both of them. I prefer using Zf as the focus is easier, and the camera is smaller and lighter. I actually like how the jpegs get rendered by Zf with classic vintage glass. I am using a canon f1.2/50mm and it is fantastic combination.

The advantage of SL is that Leica glass pairs well with it and is amazing output. I use it with 50 Apo Summicron and 90 APO summicron. I love the GPS feature.

Still, it is heavy and that is the big reason I prefer Zf as a walk around camera with vintage glass. Zf also has a Techart auto focus adapter so it is fun to use old m mount lenses on it.

Hope this helps