I agree with sleet.
I went from Tarsus to Galaxy (I'm a fan of trading) and got it fully upgraded before I got tired of lugging around Plutonium and the like.
Then I went for the Demon and have been playing with it ever since. I absolutely love it. Only thing it's missing is a turret, but when you're going that fast and pulling those turns...well, the turret wouldn't get much use anyway.
I recently decided to purchase a centurion and deck it out with full upgrades (took over a million credits...geez). To my surprise, I found myself not liking what I found (I played the original and the centurion seemed kickass back then).
Thing is, with the Demon, I can support 4 plasma guns with ease. A lot of people here are running smaller end guns so that they can fire them more often, but to be honest, even with 4 plasma guns on my Demon I don't run into too many problems.
From memory, if the afterburner is off, I can fire off two full volleys (of 4 guns each) until the energy bank drops to 1/5 or something. After that, there is some combination of guns that fire when the energy is available. The maxed reactor level generates 9000 KJ, and you figure each gun takes 1,700 KJ/shot...even adjusting for power needed to shields you still are able to fire off about 2-3 shots/second.
If the volley of plasma fire makes a direct hit on the enemy, odds are his ship is out of commission. Talons blow easy, and I've even taken out a Dralthi with a single volley (usually doesn't happen...but if you hit it just right you can do it). My style of fighting is head-to-head passes, so you figure that if it's a single ship you've got two individual chances to blast it out of the sky right then and there, no second pass. That's typically been good enough for me. But in the rare case where you miss him twice while he's coming at your face, even a single impact of plasma will deal 38 MJ of damage - about 3 times as much as your average "other" gun. The only weakness of fighting with plasma guns is that you lose out on those dizzying circular chases, but all you need to do in that case is to hit the afterburners and put some distance between you and the crazy pilot, and come around for another pass where you can use your guns to maximum effect.
The Centurion can't really support heavy guns...the energy consumption becomes a problem. Also, the Demon can run the afterburners indefinitely, whereas the Centurion will slowly lose energy.
Finally, one of my favorite things about the Demon is the positioning of its guns. It's conducive towards accurate aiming. The Centurion gives you greater spread, which is a little bit more forgiving if your aim is off in the lateral direction. However, if you are fighting a small ship you can't really count on scoring 3 or more hits (kilrathi fighters are a notable exception...here the spread is often blessing because they are thin and long). The Demon has all of its guns positioned in the center, which means that you if you score a hit, it's likely that you've scored all 4 of them. Also works much, much better with autotracking - you have to aim within the red box for the autotracking feature to be of any use (it works on unselected targets as well, but you need to get the fire somewhat close to the bearing of the vessel...it's not a FF missile). With a centurion...doesn't really work out too well (too much spread!).
One last thing about missiles...personally I tend not to use them very much (I don't know why). I use torpedos more than I do missiles (although for some reason I put a tractor beam on my Demon, so I'm not using either right now). But even then, 5-10 missiles should get you through any major fight (assuming you don't launch them willy-nilly). And there's really no advantage to having more missiles than you could reasonably use in a single fight (again, I don't use them at all at this point).
So personally, I'm sticking to my Demon. It can outrun anything that doesn't have fur. Might try a catship sometime in the future, but those only come milspec and are incredibly expensive.