Get the '18! It's basically a Q of LCI vs pre LCI, and what you intend to do with the car.
With the LCI, you get: (summary from CanAutM3 here -
https://f80.bimmerpost.com/forums/sh...8;postcount=92)
- Bi LED headlights standard (instead of Xenon)
- New look full LED headlights (optional)
- New look full LED rear light clusters
- Black ///M emblems in the seats (instead of silver)
- Fake stitching on non-leather dash
- Black 666M wheels optional with the CP
- iDrive 6.0
If and when you decide to sell it, the LCI will hold better value. Plus, you get to enjoy the factory and maintenance warranty for some time.
However, if you think you're gonna mod the car pretty quickly and heavily, you're probably better off getting a '16 as it's cheaper and you won't have to care about voiding any warranties. At 15k miles, all it needs is prob a clean PPI and you'll be good to go.
The F83 Comp is a blast, I'm lovin' it. Good luck with your search.