Bite vs Dragon Tail Gyarados

Depends on what role you want for your Gyarados. If you need a dragon attacker, DT/O. If you need a dark attacker B/C. If you are just looking for a generalist either DT/HP or B/HP are also good.

Personally, I fast TM'd my best Gyarados from DT/C to B/C simply because I could use another decent all dark attacker (all three of my L30 Tyranitars are B/SE).

Oh, and I wouldn't even think of defending. His CP was the only thing that kept Gyarados in gyms before the rework. And now we even have Zapdos who takes out Gyarados more ridiculously fast than Jolteon used to do.

EDIT: I really like Bite as a fast attack given that it is very dodge-friendly.

I ran this on pokebattler for my DT/crunch and decided to change to bite/crunch in the end. It's pretty close but bite is more dodge friendly as blue swan says. DT builds energy for crunch more quickly so it possible to get off an extra crunch using DT. Overall, I give the edge to bite. This of course is assuming you are using against psychic.

i prefer the speed of bite. esp when paired with crunch for the double dark. Only reason to keep DT is to pair with O as a dragon slayer

I change all my gyarados to bite, because I have many dragonite with double tail move.
And I guess, bite on gyarados is the fastest attack in bite moves

Bite, hands down. For all the reasons listed above and more.

Personally, I think that a bite/hp gary is one of the best generalists in the game. Before I ever came close to having a machamp, dragonite, tyranitr or any other heavy hitter... B/HP gary was my dedicted snorlax and blissey killer.

I always had two maxed out, a 100% and a 95%, Their names are Gary Uno and Gary Dos. They still see a lot of action.