Never cared for the space gray in the 6 series but in fairness, it does hide oily smudges better than say matte black. White, gray, and silver can resist them better than darker colors. But other than that, I prefer the matte black and jet black 7/7+ based purely on aesthetics.
Space gray was the solution to the slate iPhone 5's Scuffgate. Honestly, space gray in 5s and SE might be the last great design I ever liked from an iPhone. Haven't really cared about an iPhone design since 2013. HTC surpassed Apple in design by 2013. Samsung surpassed them all by 2015.
No antenna lines and black top/bottom made this 5s the final classiest rear design I ever saw from Apple. This or the silver 5s/SE...
Don't really care for the screen size of the 5s or SE though. A "4.5 display is my bare minimum. The 5 series was also too boxy. The 6 series might feel slippery but rest comfortably when gripped and no case.
I still like the glacial silver HTC One M7 design more than any color of the 5s and SE. The Man o' War of smartphone touchscreen design was the original iPhone. Nobody could catch up to its design until maybe the Nexus One or iPhone 4 in 2010. Okay, maybe the Xperia X1 in 2008.
Then I felt Nokia came out with the N9 in 2011 that blew them all away. In 2012, I didn't like many designs except for maybe the Nexus 4, HTC Windows 8, and Sony Xperia S. Then HTC took over with the M7 and M8 before Samsung ruled with S6 edge, S6 edge+, S7 edge, and Note7 during the past 18 months.
The last three iPhones from 6 to 7 have average designs to me. The 5s is great and maybe a favorite to many users here, but I have seen better and held more comfortable phones over the years. Can no longer text fast or play Plants v Zombies 2 without messing up on that tiny "4 display. Nice designs but no thanks, 5s or SE.