Why not both?
Kinetic weapons have advantages: lots of kinetic energy, and simple. And the clear disad: ammo is heavy.
Beam energy weapons have similar advantages, along with straight line of sight all the way to end of range. They also have disads: reflectability is probably the worst. Being able to be defeated by a piece of tinfoil kinda sucks. This issue (among others) means such weapons are more likely to be used ship-to-ship, than in an infantry role.
Non-beam energy weapons have advantages: lots of electromagnetic or thermal energy. This disrupts electronics, including shields and countermeasures. They also have disadvantages: no real penetration beyond what they eat away while destroying the plasma; and no easy way to "carry" the plasma to the target.
So if you're firing plasma, then it arguably "makes sense" for the plasma balls to have a small solid kinetic core to "carry" the plasma to the target.
This answers the argument of "what if they run out of power"? Then they're down to "mere" explosive-kinetic flechettes, still effective against soft targets lacking kinetic armor.
But while they have power, they are firing those flechettes surrounded by a plasma toroid, keeping it stable over longer distances, preventing it slowing, and delivering a plasma-energy punch at the far end. The hard flechette breaches soft or lightly-armored targets, brittle asbestos and carbon fiber that plasma would have a hard time with, allowing the softer plasma to penetrate in its wake.
Edit: there's been confusion over this suggestion, so I'll try to re-explain.
I am proposing that the weapons fire a bolt of plasma with a solid core.
This is to address the problem that real-life "plasma" weapons have: that they simply can't be directed through the air without a charge-carrier connecting the weapon to the target. Real-life plasma-weapon designs are essentially like a taser, where the wire vaporizes into plasma (in the sense of "electrically conductive gas"), and then the plasma carries the charge. In this way, the wire works kind of like the stepped leader in a lightning bolt.
This is very unlike the traditional "laser bolt" from sci-fi, though, which is a projectile, not a beam weapon. But you just can't fire a charged gas like a projectile, through the air. The air resistance stops it and dissipates the charge within a foot or two.
So instead, I suggest a small, very-highly-charged metal object (a "flechette"), so highly charged that the air around it becomes plasma? This is almost certainly also infeasible, but at least appears more scientifically believable than a weapon that's purely made from lasers, or puffs of plasma. To an observer, all that's seen is a bolt of superheated glowing air.
The metal carrier at the center of the bolt penetrates armor and conducts the surrounding plasma charge straight into the victim, where it disintegrates, electrocutes, cooks, destroys electronics, melts, welds, etc... basically like a small lightning strike.
The advantage of this over regular projectile weapons is that the ammo becomes an awful lot lighter, as it delivers more damage per gram because of the strong charge it carries, to the projectiles can be smaller.