Well, I don't own a 6s (plus) but I did preorder a silver 6s Plus as soon as I could when the window opened. I own a silver 6 Plus and I had a bit of remorse over spending near a grand on another phone so soon, so I cancelled.
My reason for choosing silver was... Well, to be honest, I wanted gold when I got my 6 Plus. The local Verizon store didn't have one in stock and neither did any other Verizon stores in my area... No apple store within 2 hours of driving range. I had to go silver. My previous phone was black so I wanted some change.
I guess it just grew on me as I kept using it. Eventually I came to appreciate how 'icy' and 'sleek' it looked, I really love the 'cold' look it has. It looks just so seamless...
Gold is pretty, rose gold was too much for me to be honest, and I do like space grey color - it just looks so chic! If my previous phone wasn't black, I would have gone with space grey for sure.
I know I almost got a silver 6s Plus but when iPhone 7(or whatever it will be called) comes out, I'll probably get space grey, unless a new color sells me (if one comes out) or other colors really stand out due to the design. I personally hope for something bluish, as I like blue - like a rose gold counterpart? Probably won't happen though.