MathWing perspective:
The Firespray is more durable on a PS1 cost adjusted basis: normalied to a TIE Fighter, about 2.57x for the Firespray vs about 2.15x for the Defender.
This results in the Defender having the worst jousting efficiency in the game at about 80%, vs 85% for the Firespray, and for reference, 81% for the TIE Advanced.
The rear arc on the Firespray is amazing, as are its upgrades. But I still don't know about the white K-turn, so I'm undecided. I would probably, at this point, generally lean towards the Firespray. The PS6 Defender's named ability is really nice though. As long as you have a lower PS ship that has TL on it, you can get much improved damage out of him. For 5 points more over a Delta Squadron Pilot you really can't go wrong, if you were determined to have a Defender to begin with.
To completely fly in the face of the OP's original question, here's a squad that uses both.
Carnor Jax + Push the Limit (29)
Colonel Vessery (35)
Bounty Hunter + Weapons Engineer (36)
A more redundant (and probably better) idea is:
Soontir Fel + Push the Limit + Hull Upgrade + Targeting Computer (35)
Colonel Vessery (35)
Omicron Group Pilot + Fire Control System + Weapons Engineer + Engine Upgrade (30)
I actually like the idea of this 2nd build, even though it's still not as durable as many other Imperial lists. I don't think the loss of a Gunner on OGP is that significant, since the Shuttle should be firing with at least TL every turn, and many times TL + F.
I'm not really feeling the Delta Squadron Pilots so much. But if you had exactly 30 points left over then you could squeeze in a Defender, rather than having to spend 33 points on a Firespray, as Vorpal said.
Edited by MajorJuggler