(to the Defender)
There's a few threads out there lately in which people are interested in benchmarking the TIE Defender against the Firespray.
This is unfair, because the TIE Defender is completely outclassed.
TL;DR: the only reasons you would ever reasonably choose a Defender over a Firespray:
i) you only have enough points for a Defender;
ii) you want a more effective blocker;
iii) aesthetic appeal;
iv) you want a cheaper ship dollar-wise;
v) you want a less easy-to-block ship.
All of these (except perhaps iii, because those Defenders are sexy lookers) can be satisfied with other ships, so this could also be titled "Anything the Defender can do something else can do better".
I will not reiterate Major Juggler's MathWing results that tell you that the TIE Defender is one of the worst jousters in the game for its price. He's posted that several times already, I'm sure we're all familiar with it.
Instead, I'll be focusing on its ability to move and fire, which has been brought up in context with the Firespray.
A staple of the arguments which suggest it's a good choice to pick the Defender over the Firespray is the idea that, with its white 4 K-turn, the Defender has the near-equivalent of a rear firing arc. Near-equivalent, because proponents seem to feel that being able to use it for a secondary weapon shot gives it the edge.
This is simply the wrong assessment.

From a given starting point, this is the range of modifiable shots you can take with the TIE Defender in the pseudo-rear firing arc. If you barrel roll to soften your need to predict, your range of shots becomes this:

And this is nice, assuming you are able to pull off the K-turn/barrel roll without being blocked.
That's kind of a big deal. The Defender only has one K-turn, and it's speed 4. I speculate here (since my K-turn use on the Defender has not yet been blocked) that in a situation where you think it will be useful to K-turn and get a shot off, your opponent will also get an idea that that's what you're after. The limited speed options makes avoiding (to force a barrel roll, which equals an unmodified shot) or blocking it trivial-- and if you do get blocked, you do not turn around. No rear arc for you.
A Firespray's rear arc cannot get blocked. If that alone does not convince you that it's far better than the Defender's 4 K-turn, I'll put some harder evidence out there.
Here's a picture showing you with merely the banking and turning 1-and-2-speed maneuvers where its ranges lie:

Having the option to change the angle of that rear arc is crucial. Even at speeds limited to 1 and 2, the Firespray's rear arc has clear superiority with the primary weapon.
And, to wit, if you are into taking unmodified secondary weapon shots with your Defender using barrel rolls, we have to allow the option for the Firespray to take its unmodified secondary weapon shot using its two speeds of red K-turns:

The plain Defender's only saving grace is one arc of shots that the Firespray can't get: a secondary weapon shot with 4 dice and the ability to have focused or target-locked prior to shooting it; even then, you must be using the HLC in order to have a dice advantage over the Firespray's primary with that shot. Price-wise, you've now upped the PS 1 Defender to 4 points more than the BH, and what have you gotten for it? -2 PS, -1 shields, -3 hull, +1 agility, +1 attack (except at range 1), no +1 evade to targets at Range 3, -90% of the rear firing arc, 75% smaller base, +barrel rolls, -evasive actions. Is it worth it?
Colonel Vessery is a different story. His auto-target lock on a pre-target locked target is a superb ability. However, one good pilot does not redeem a ship; it makes a niche pilot. We've seen this with Vader.
So we've cleared up (I hope) the idea that a white 4 K-turn is anywhere close to making the Defender comparable to the Firespray in terms of ability to attack.
Now we're going to talk about dials.
We'll start with the non-red maneuvers, shown side-by-side from the same front edge starting point.
It surprised me how similar they were. (I did not include barrel rolls because that would have been prohibitively time-consuming) Specifically, their top speed maneuvers net them the same distance, and they have very similar lateral movement patterns, even in the white sections. But now, suppose we want to clear the stress (since these are non-red maneuvers, presumably we are stressed). Here's the green maneuvers from the same starting point.

Perhaps, simply by glancing at the dial, you would have guessed that the Defender is really hampered by stress. And you'd be right. If the Defender gets stressed, it can only go straight if it wants to clear it. Not only that, but the Firespray has the option to access banked shots with green maneuvers; this means that, against an enemy to the side, the stressed Defender pilot is forced to choose between the ability to modify his dice or the ability to take a shot this round at all, which is not a choice the Firespray is ever faced with.
Seeing the maneuvers it can and can't do while stressed in the previous picture still makes it seem not too bad; it can still turn and it can go fast. Faster than the Firespray, even if it wants to clear stress, so on a far-away ship it can easily close, focus or TL, shoot at better range than the Firespray. Not so bad at all.
But if you think that way, as I used to, consider this example which actually happened to me: I was directly approaching the edge of the board, and used Opportunist for a 5-shot Range 1 primary attack (focused, and TL'ed from Vessery's ability) from the side. Finished off the target, but then... I eyeball the distance and guess that a 3 turn will put me off the edge. This situation spelled pure trouble.
There is only one route you can take on the Defender dial to survive.

If a 3 turn sends you off, you can only 1 bank and then 3 turn. Clearing your stress isn't an option until the third maneuver, because these are both white.
Was this a great position to get myself into? No. It was worth it to take out the E-Wing, but then I had to wait quite a while for my "heavy hitter" to get back into the fray. Fortunately there were no other ships around, and likewise fortunate that the target didn't survive.
Would the Firespray have had the same issue?

No. Its 2 turn is white.
The ability to only clear your stress by going straight can, under the right circumstances, be incredibly prohibitive. The Defender's dial certainly likes to make those "right circumstances" easy to stumble across.
So I conclude that in nearly every way, the TIE Defender is completely outclassed by the Firespray. There are a very select few things that it can do that the Firespray doesn't. And Colonel Vessery is a nice pilot who could find a niche in some Imperial builds, depending of course on the ability to acquire/maintain target locks (Expert Handling and/or Wes Janson on the enemy side? While Vessery isn't 35+ points of dead weight without his ability triggering, he's not too far from it).
My personal thoughts? FFG made the sharp turns red because they were planning on giving the Defender the ability to use its short sharp turns like K-turns, e.g., have the option to flip around after completing them. They nixed the idea and then forgot to take the red off the dial, leaving us with a ship that does nothing to redefine the meta or even eke out a place where it belongs uncontested.