Which Civic would you pick?

MILEAGE ALONE IS NOT AN ADEQUATE INDICATOR OF CONDITION. Without doing a thorough pre-buy inspection and test dtive of both I wouldn't pick either.  

The 2015 Civic is the better option.

I'm not a fan of any Civics built after 2005. I wouldn't get either. However, if I had to pick between the two, there are two factors I'd consider - which one is more maxed out when it comes to depreciation and which has been better taken care of by the previous owner (maintenance records would be helpful). 

You can't do a proper mileage comparison unless you can tell whether most of the miles logged are city or highway. The 2015 model could have been spending most of it's running time in bumper to bumper traffic on short drives while the 2017 model cruised the highway mainly. I'll take a vehicle which has been mainly used for highway commutes and properly maintained over a vehicle that goes through short cycle drives in bumper to bumper traffic. As a bonus, you can still use mileage as a haggling point.