Thought about getting a Mamba last year. I'm 5'7" on a really good day, but with short legs and long arms/torso. The 15.5 was decent in standover but really cramped for me in reach. The 17.5 felt good when I was rolling on it but I couldn't even straddle the bloody thing. Okay, that did not sound good
I'd be wary about messing with the saddle fore/aft like Zephyr was saying. You could mess yourself up biomechanically doing that.
Standover height definitely does matter for sure. I'm still riding my old 26er for a reason.