I've never used a biothane harness, but I do use biothane!
From the research I've found on it, it's stronger than leather. From personal experience, it's very durable, definitely doesn't absorb water, sweat or grime, and wipes clean. It doesn't get that 'used' look as much.
I haven't found mine has stretched at all.
I hope what I worte made sense. I realllllllyy need to go to bed.
...you will be more disappointed by the things you didn’t do than by the ones you did do. ... Explore. Dream. Discover.”