I am working on building a somewhat-near-future(500 years, but after near-total-annihilation and then induction into a more advanced galactic society) in which infantry-based wars are fought throughout the galaxy, mainly by humans. Due to the inherent danger of fighting in a world where FTL ships and nuclear fusion reactors exist in large quantities, wars will be strictly regulated by a super-advanced autonomous group. This group ensures that opposing armies are relatively equal in power, and with the kind of technology that will lead to an interesting fight. The best army, then, is not the one with the most money, soldiers, or technology, but the one that's able to accurately predict what tools work best in the context of the location of the battle and the equipment being used by the enemy.
Thus, I want to know what pros and cons laser and projectile weaponry bring to the table. The kinds of things I'm mostly interested in is what planetary conditions or tactical situations each would excel or be hindered, and what kinds of armor or fortifications soldiers would use to protect themselves from such weapons. Perhaps also the expected size, shape, maintenance of the weapons. Basically, why would you use one over the other, and why you would later use the other over the one.
By 'projectile weapons', I mean the somewhat-distant-future of modern guns, with some sort of explosion in the barrel delivering force to a small bit of metal that then flies out of the barrel and into the enemy. By 'laser weapons', I mean something that fires a beam of energy with the capability of causing fatal harm to someone. I'm hoping for an explanation of modern laser technology, but with the size, weight, and usefulness of a modern rifle. This is something I don't have much knowledge on, so an explanation of exactly what would happen to a laser-shot victim and in what timeframe would also be appreciated.
Bonus points for a scientific explanation of plasma weaponry, if that's different than laser weaponry.