I've been using the same roo strop for what must be close to 3 years now if not a bit longer. I can say without a doubt that if you look after it (as in don't do anything silly to it) kangaroo will last you many many years. I've never had a cupping issue because it has always hung in the bathroom - I've never rolled it up. It still works as well today as the day it was made.
My point is that IMO you will not find a more durable strop leather than kangaroo leather and if you treat it well it will last you many years. I fully expect mine to last me to the grave, despite how thin it is.
Another consideration is that, according to a recent post by MickR (who knows about these things), Australian customs are cracking down on backyard kangaroo exports, including leather. So you guys in the US are very lucky to have SRD supplying roo strops.
James.