I have both sheepskin and ThinLine seat savers for my English saddles, but stick to the ThinLine because I found the sheepskin I have to be too bulky and create a pressure point under my thigh where the edge is, though this may just be an issue with my specific one (an older model Fleeceworks). If it weren’t for that, I would probably layer the two and it would be perfect (I’ve tried this just sitting in the saddle and really liked it aside from the pressure point issue). The ThinLine is very low profile, but adds a surprising amount of cushion/shock absorption. I like the fact that it’s thin and so it doesn’t make my saddle feel smaller or make me feel perched, but it does make my older, hard seated saddle softer for trail rides and keeps me comfortable for longer. It does not have the same amount of cushion as something thicker, but makes a noticeable difference for me. I believe they make Western seat savers as well.
One thing to take note of if you get a ThinLine for an English saddle is the sizing, as the close contact, all purpose, and dressage ones are all different lengths and will fit very differently regardless of what style your saddle is if you have a deep seat and/or a larger seat size. I ended up getting a dressage one (19.5" long) for my 18" deep seat jump saddle and it barely fits (the close contact one I originally ordered was way too short, and the all purpose is slightly shorter than the dressage).