I would go for the 15/14/14. But do they both have the moveset you're looking for. If not how many TMs do you have and would this be a problem? If you're low on TMs and only one has the right moveset power up that one, they're nearly identical. Also wow, my best latios based on IVs is only 91%, congrats.
Which 96% Latios should I power up?
The first Latios is 15/14/14
The second is 15/15/13
I realize it's probably nitpicking, but I'm wondering if there's any secret math that would determine which one is slightly more useful.
Answers
I'm not sure of any difference. I caught a 15a/13d/15s the first day or two and I have it at level 27 giving it some dust whenever I feel like it. Didn't really backfire bc the other bests I caught were boosted 15/15/10 and boosted 15/13/12. Anything 15a and 96% I count as good enough for some attention.
Except that the OP is not talking about pouring rare candy in it to boost it. What if the OP has done so many Latios raids that (s)he can boost it without using rare candy?
I would boost the 15/15/13 (allthough I don't think that there is a significant difference in performance). However, only if I would not have to spend any rare candy on it as I have two DB/DC Dragonite already and more of the same would not really help me out.