I guess it would depend on if I'm going to be cutting the shape of the item I'm making or if I am preparing a pattern for the design on the piece. If I am making a cutting pattern, I would use something sturdy that lasts. I've used aluminum flashing shaped with tin snips, and I've used Cereal box cardboard.
If I am preparing a design, I like SEE Through material. The reason is it makes it easier to place and trace the design correctly. For this I use transparencies like Teachers used to use, as well as sheet protectors that can be opened out to make a bigger piece. I just print the design, lay my clear over the top and trace with a permanent marker. Then I lay that over the leather and trace again with a stylus or with a dulled pencil or empty ball point pen. I can tape it down and not tear the paper when it's removed. I can pin through it, if placement is critical.
I also have some vinyl, but my vinyl is clear and a few mils thick. I get mine from my local True Value, for about $5-$6 a linear yard. (it's 48" wide I think)