Who's a better fit with a 6 ship swarm list?

Vader (34)

TIE/x1

ATC

Engine Upgrade

Soontir (34)

Royal Gaurd

PtL

Autothrusters

TC

Both should do amazingly. Very hard choice.

Flip a coin?

What's in the rest of the swarm?

As noted, you pays your money and takes your choice - the only time I think it makes a difference would be if you planned to remove the Engine Upgrade from Darth Vader (in favour of Shield or Hull upgrades) and stick him in with the swarm, with Swarm Tactics. In the configurations as described, both should work fine.

If you're planning on running your Ace in the swarm, I'd say Vader - tankier, a dial that maneuvers more like a Tie. If you're using them as a flanker, I'd say Fel, since trying to use him as a swarm member is a waste, but as a flanker he's terrifying. Not that Vader's a bad flanker, but he's not nearly as zippy as a squint.

What's in the rest of the swarm?

As noted, you pays your money and takes your choice - the only time I think it makes a difference would be if you planned to remove the Engine Upgrade from Darth Vader (in favour of Shield or Hull upgrades) and stick him in with the swarm, with Swarm Tactics. In the configurations as described, both should work fine.

Howlrunner plus 4 Academy pilots.

I agree with the above.

Vader with the swarm, or Fel if you want a flanker.