i like the space gray looks cool and i have one. i also have a friend and they have the gold one looks classy. its hard to think which one to buy but for me i will choose the space gray. looks great.
Bought gold first and then for some reason I had to exchange to grey iPhone 6 I must say all that time I was missing white front and standout bling thing . So I am now back to gold. Gold also has better demand in the market making it more precious at resale point. To me grey iPhone tends to hide while gold shines and stand out.