I’ll let some of the more well versed folks fill in the blanks, but the basics I’ve gleaned from hanging out here are:
Balsa strops are very firm, for the times rounding the edge may not be desirable. Standard cow leather is a good all-around and fairly inexpensive. Kangaroo leather is extremely dense grain and gives an excellent polish, as well as having more of a burnishing and polish effect without any paste or spray on it at all. Nano cloth is the opposite, it has no inherent polishing effect so when you pair it with very fine sprays or paste you’re getting only the abrasives and not the substrate making scratches.
I’m pondering strops too for my first major upgrade to my kit. Always a fun topic around these parts!