Willie, both seemed ok, but the dogbane is definately stronger. Of those 4 bows I mentioned 2 have failed and 2 are still alive. The 2 that failed were tiller issues. The sisal bow that broke ripped right through the backing, but the dogbane one still had an intact back, just sort of folded as the wood failed. Not extensive in comparison, but the dogbane is certainly stronger.
These bows were backed not for performance, but rather as insurance against my rookie bow making abilities. They were also board bows and the 2 failures were both red oak.
I started a thread about these backings a while back and there were some suggestions on sisal by Joachim using hide glue and inducing reflex with sisal as it stretches more than the dogbane would. This might have some performance gains? Can't say I have enough experience with it to say though.