I am 6' 1" and ride a Soma Rush--- went with a 57--- and it effectively runs large. I cannot imagine a 59--- it would definitely be too large for me. Consider the geometry--- with the higher BB, the TT is set relatively low (considering the seat tube is measued to the top of the tube) to offer a decent standover, and with a relatively short headtube, unless you go with a rising stem, you should end up with a lot of reach. My 57 has a longer TT than any of my other bikes (a 57, 58, and 59). I would go for a 57--- mine fits perfect.
I had similar deliberations after reading the size chart--- 57 vs 59. I went with my gut--- and have no regrets.
I have no idea why Sau's bike is so "twitchy" or "heavy"--- mine rides like a dream.