Particle covers are reactive balls with a particle additive. Particles are typically microscopic glass beads, they could be other materials. Think of 'em like treads on snow tires. The particles provide more traction in oil and tend not to jump off the dry as much as reactives.
One isn't necessarily better than the other, it's all in the type of reaction you're going for. Particle balls tend to work better in heavier oil as the particles provide more traction, but a lot of new high-powered reactives give particles a run for their money. If you took a reactive ball and added particles to it, it'd be able to handle more oil.
Some people like particle balls because they are usually smoother than reactives, especially on over-under conditions (where the ball hooks too much or not enough with a small difference in line). Others like reactives because the increased backend and angle can make it easier to move further inside and still carry.
Higher load particles are usually smoother than lower-load particles. Again, the treads on a snow tire analogy is useful. Think larger treads for higher loads and smaller treads for lower loads. Low-load particles can react very much like solid reactives in terms of backend. A higher-load particle ball will almost certainly burn up if there's not a lot of head oil. Pretty much all the high-load particles left are heavy-oil balls. The GP2, Super Carbide Bomb, and AMB Particle are some of the strongest heavy-oil balls ever made.
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