Plasma vs rail.
Plasma is more powerful at penetrating armor than magnetic weapons in some cases, but I think that a railgun would be better. It's fast. Really fast. Plasma's just burny (but only really burny). Right?
Basically what you're asking here is "thermal vs. kinetic". Plasma weapons rely on heat to hurt things, and as such are great for systems that have low thermal conductivity (such as say, things in a vacuum). On the other hand railguns have a huge amount more penetration force due to actually being a kinetic impact. Plasma works great against things that can crispify (like human flesh) while railguns will literally just punch through a soft enough target and pass out the other side without really causing any serious damage. (Seriously, a fast enough railgun projectile would literally pass straight through a person leaving two fairly neat little holes behind without any other real damage, while a plasma shot would basically crispify you to death). Railgun shots fly faster (so fast that they can actually go off course when they hit the atmosphere) than plasma shots do, which aids in firing over long distances.
Considering space warfare, though, they both kinda suck. Both are way too slow to cover any meaningful distance (heck,
light is too slow in anything but the absolute most close range shots), and have the disadvantage of both having mass which gives them a hell of a stronger backwards kick than a laser beam does; a backwards kick that you have to burn an equal amount of momentum in the opposite direction or be pushed off course by.
Realistically any sort of spaceship warfare with anything moving at a speed less than the speed of light simply comes down the question of who can burn the most delta-v and still have enough fuel left to get to wherever they want to go. The distances in space are so vast that there's plenty of time for anyone to see your shot coming and move out of the road if they want to (since you have to be shooting at not where they are, but where they are going to be in several minutes/hours), so the question just comes down to how much each of you can afford to move out of the road and not end up stranded in the middle of space. Lasers still have the problem in that you have to know where someone is going to be in another half hour or so, but have the advantage that it's impossible for them to see them coming, because the information that you fired at them reaches them at the same time that the laser does.
Stealth fields:
These are actually pretty simple to make (and honestly we're approaching the technology level where we could manufacture one right now), there's just a few big problems with them.
1) They'd actually be a "stealth
shield", not a "stealth
field". As in you'd have a physical shield surrounding you that bends light to conceal whatever is inside of the egg-shaped shield bubble.
2) If no light gets into the shield, then you are essentially flying blind. If light can't get in then it can't get out either, meaning that you can't see where you're going. (On potential solution to this would be to make it only work on certain wavelengths, though that means you wouldn't be invisible to anyone looking in on the same wavelength that you were looking out on).
3) Due to physics any sort of propulsion system has to be outside of the bubble (though you could open/close the bubble as needed with some work), meaning that you can't steer while invisible either.
The end result is that while you
can go totally invisible, you've essentially got to surround your spaceship with a bubble made out of a very delicate metamaterial that probably won't work well if it gets damaged even a little bit, and while it's active you have no idea where you are going nor can you turn in any way. This also gives the further disadvantage that if an enemy sees you go invisible than your invisibility doesn't make a difference, all they need to do is calculate out your trajectory based on your speed when you cloaked and then put a bullet right where you are going to be, and there's nothing you can do to dodge it or even see them shooting at you.
Cause energy is literaly free and simple to harness and permeates every thing.
While this is technically true (energy/mass equivalence and all that jazz), in reality it's impossible to extract the energy from 99.9% of those things.