Obviously the 91 %. 14/ 15 attack stat wont make that much of a difference in your case. Still if you have enough candies you should evolve both to pupitar and check their iv again or try a single power up and check. You might be lucky to have increased ivs. I was lucky this way plenty of times.
Which Larvitar should I evolve?
1) 506 CP: 91 IV with 14 A, 15 D, 12 S
2) 500 CP: 84 IV with 15 A, 12 or 11 D, 12 or 11 S
It would be preferable to get your Tyranitars from raids, but I assume you're asking because that's not possible for you.
Tyranitar does have a 15 attack only breakpoint against Entei at level 40, but that's not terribly relevant. I would go for the 91%, 14 attack will require 0-1 more powerups to reach breakpoints compared to 15 attack.