Here's my opinion regarding TTar v. gyara-checks/counters:
Porygon-2: Tyraniboah uses this as set up fodder, porygon 2 is tyraniboah's bitch. CB ttar cannot kill p-2 with pursuit, and p-2 is generally heavily ev'd for physical defense.
Celebi: Either way, Celebi is more likely to stay in and Grass Knot than flee. With Celebi's usual investment into physical defense, Tyraniboah's Dark Pulse does very heavy damage. Either way, neither loses to Celebi, but neither can take it down without taking heavy damage from grass knot.
Rotom-A: Rotom A stays in on TTar 80%+ time. It always stays in to set up reflect or w-o-w. Both of these will screw over CB ttar. Tyraniboah on the other hand will destroy it with Dark Pulse.
Jolteon: A Specs Jolt locked into Thunderbolt is not going to run, which means it'll be leaving a huge dent on CB ttar before it goes down. Boah on the other hand can still manage to use its substitute strategy thanks to leftovers. If you got Focus Punch, of course you have the advantage. A specs Jolt locked into Shadowball/hp ice is toast facing either ttar, either giving boah a free sub with which to reek havoc, or getting pursuited to death (honestly though getting locked into pursuit sucks).
Against Sub-Pass Jolt, Boah is unquestionably superior though, setting up subs with impunity against it's weak bolt-beams that can't even break sub. At the very worst case scenario (boah switches in on jolt's sub), it can set up its own sub as jolteon passes, then break the enemy's sub as they break boah's. CB ttar on the other hand, can't touch Jolteon at all as Baton Pass negates pursuit. :/
Against Zapdos-- Boah's Ice Beam hurts Zappy a lot while most zappy can't break substitute with any of its attacks. The only problem is status, though Roost + 3 attack Zapdos is set-up fodder for Boah. CB tar can only rely on the 70% acc. Stone Edge, though this can 1hko zappy. Which one is better depends a bit on Zapdos' set.
Latias-- Ok, CB is definitely better here.
Starmie-- I wouldn't advise trying to counter starmie with Tyranitar, no matter which one it is. In a 1 on 1 though, it's like Celebi-- Starmie probably won't run fearing pursuit, and is 1hko'd by any dark attack from any ttar-- problem is that it will always be leaving a huge dent on ttar regardless of its set.
Gengar-- Without a doubt, the best TTar to have here is an extremely special defensive bulk CB ttar. Standard CB tar and Boah are both 1hko'd by LO focus blast. Gengar loses to either without LO though.
Cresselia-- Both destroy cresselia but are likely to eat a thunder-wave. Boah handles reflect/psycho-shift versions better but has some minor issues with Calm Mind versions (calm mind cress still loses to boah the vast majority of the time).
There's some others but yeah, both sets are very useful, mostly because Gyarados' biggest enemies are frail special attackers, often with a dark weakness making TTar an obvious answer.
Boah handles P-2 and Rotom much better, but the same can be said about CB ttar in regards to Latias in particular, and Zapdos too generally. Of course, both being tyranitar, Boah can still kill latias most of the time and handles zappy pretty well too, while CB ttar obviously has the potential to put P-2 and Rotom in pain.
Picking for your team is of course important. If you have a CB scizor on your team of course Boah becomes more attractive, while if you have a Sub-Heatran CBtar will bring a better balance.