I play guitar and bass and have for longer than I care to talk about. A couple things about guitar straps from my point of view. In my opinion there are two types of straps, ones that slide, lined or rough side not against me and ones that will not slide, rough side of the leather against my body or clothes. For me that is the first thing I look at. If I am putting a strap on a Les Paul it can slide, the body of the guitar is so heavy it will not move around and try to "neck dive". I have a Washburn 6 string bass and I have to use a strap that stops the "neck dive" or I lose the bass anytime I am not literally holding it up with my left hand and I have better things to do with that hand. Acoustic guitars will do this too. Almost all of my bass straps are not only lined but padded, basses are heavy and I am not sure what they are lined and padded with but the combination works to stop the neck dive and the lined side is smooth. These straps were purchased retail and they were expensive, $55-$60 or so for each one and they are so consistent that I believe they are made by a machine. I can snap a picture of one if you want to see it.
My favorite guitar strap is almost 35 years old, plain Jane adjustable leather strap. I can flip it over for rough against me or smooth depending on the guitar I put it on. The leather has always been super soft and has not stretched or shrunk the whole time I have owned it. I have sweat through it many times to the point that I have literally squeezed sweat out of it. It has never stained me or my clothes. Again I can post a picture if you want to see it.
Edit to add I would pay double what I paid for those padded straps for one more plain Jane that is like the one I described above. Once I figure out what the leather is and what dye was used on it I will make one just like it.