Here's some things I'd consider:
Both mileages are completely reasonable, KBB puts the average 2014 BRZ mileage at 44,000.
https://www.kbb.com/subaru/brz/2014/...ntent=buy-used
As far as can be figured from the pictures, looks like they've both been taken care of but the 2015 definitely has some sort of aftermarket exhaust on it, which could be a fix/change (read: more money) if you're not into that sort of thing or don't like the one they've put on it.
The one at the dealer has been there a long time (3 months almost) so you may be able to haggle a better deal, although with only 12k miles I am not sure how much better. My car is a 13 with 21k miles, and because of our crappy winter up here the dealer couldn't sell it and were trying to take it to auction and also dropped the price $2600 before I got out to look at it.
Both are below the market value as determined by car gurus but you might also punch in the numbers on KBB and see which is better.
I'd have some questions as to why they've both had 2 or 3 owners though. Get the carfax before you buy!