Dark is crucial for soloing Gengar and especially Alakazam; both are great for Jynx. Which type is more common for legendaries: Psychic (which Dark destroys with bulk) or Flying (which Rock reigns supreme against)? There are a few good gym defenders weak to Dark (notably Exeggutor, Metagross and Slowbro), but Rock is a better gym generalist. As for gym defence and tier 4 raids, both are equally useless. Another consideration is how much worse Golem and Houndoom are compared to the respective Tyranitars.
I have a level 30 Dark Tyranitar (13/15/13) and three level 20 Larvitars: 12/15/15, 11/15/12 and 11/14/12. Hope I (and you) will catch more wonders on 16 June!
Honestly I think Rock Tyranitar will be more valuable than Dark Tyranitar. Rock Tyranitar DPS is so much Higher than Rock Golem compared to Dark Tyranitar and Dark Houndoom. With Houndooms so easy to find, even boosted, my team consist of 2 lvl 40 Tyranitars and the rest lvl 35 houndooms whenever I go against Mewtwo and I still get +2 Poke balls in a group of 12.
I don't know if going for double rock during the event is a thing. It seems like we might get one option for each move, if it comes with Smack Down, then double rock is the only possibility, and as has been suggested, if it comes with either Dark Pulse or Foul Play as an alternative to Crunch, then we have to pick after the fact with TMs. But if the only time to get DP or FP is during CD, then by all means get as many as you can.
A strategy a trainer in my area has suggested is to go for double rock on community day, you can always TM your current B/SE ttars to double dark after the event.
B/C ttar is still the highest TDO dark out there (also quite high on DPS). While B/FP or B/DP would be an improvement, B/C is a readily available fallback. You won't be able to fall back to a sub-optimal double rock ttar moveset.
