They're both good abilities, but I tend to go for Flame Body. With Flame Body, I can switch in on predicted U-Turns and burn something like a Rachi, or Scizor which means although they get switch advantage, it's at the cost of crippling their physical attacker.
Yeah. That's what I thought too. If Ulgamoth is in killing range of Lucario's Extremespeed I tend to keep it in and attempt to activate burnhax. Anyway, I wouldn't use Swarm because of this: Once it has enough BD boosts nothing can hurt it with Special Attacks outside of a critical hit which should KO. Physical attacks rarely manage to get it into Swarm range because most of them KO due to the moth's atrocious defense stat.