This is hard to choose. I absolutely love the look of the original black iPhone 5 instead of the "space grey" 5S. However, I prefer the ringed home button rather than the rounded square.
However, I think I'd actually prefer the iPhone 6 design (even though I hated the antenna lines or whatever they are at first) if it came in that same darker black as the iPhone 5.
I loved the 4/4S design so much when it first was released but once I had a 4S for 2 years and saw them around all the time, I'm kinda sick of it.
I guess in the end, the iPhone 6 in the iPhone 5 black would be my ultimate choice.