I got the silver because that was the only one they had but I slightly prefer the grey cause it has less breaks in the design. I don't like that you can't polish the grey so the silver is probably right for me in the long run.
When/if I decide to get the iPhone X is will be in the darkest color possible. The Space Gray of the X is darker than the Space Gray of the 6s Plus, but not as dark as the Jet Black, so that color will have to do.
...and the back seemed darker to me than the Silver 8 Plus (though may have just been the lighting).
I thought the same thing when I looked at it in person, almost more of a grey than a silver. It just looked a little off to me, so I went with Space Grey. No regrets.
The carrier store I went to had the Silver on display, that made me decide to go with the Space Gray. Once I opened my Space Gray, I knew I made the right decision; I really like the look of the Space Gray, particularly the coloring of the stainless steel frame.
I personally wasn't feeling the Silver as the Space Grey feels like it matches all the way around from screen to body. Once I got it and compared it to my wifes Silver, I felt 200% better about my choice.