Damage
The Spiker and Plasma Rifle have similar accuracy and rates of fire, so the difference in killing power between the two weapons is solely based on the damage per shot. The Spiker does 9.0 damage per shot to both shields and health. The Plasma Rifle does 14.5 damage per shot to shields and 3.5 damage per shot to health. This makes the Plasma Rifle extremely effective against shielded enemies, but not very useful against enemies that do not use shields. The Spiker is a more-rounded weapon.
Reloading
Unusual for a Covenant weapon, the Spiker needs to be reloaded and holds 40 shots per clip. Due to the rapid rate of fire, this depletes far more quickly than most other weapons. The Plasma Rifle never needs to be reloaded. The Plasma Rifle does have a heat gauge that fills when firing and gradually empties when not in use. Properly used, the Plasma Rifle does not overheat and never needs to be reloaded.
Ammo
The downside to the Plasma Rifle not being reloadable is that you are restricted to the amount of ammo it has when you pick it up. You need to replace the entire Plasma Rifle rather than try to pick up ammo. The Spiker uses normal ammo and you can get more ammo by walking over Spikers dropped by slain enemies. Spikers are not as common as many other types of weapons and use unique ammo. You are dependent on the number of Brutes that carry Spikers in a level for keeping your weapon supplied. Plasma Rifles, on the other hand, are fairly common since they are not restricted to one type of enemy.
Effectiveness
Both weapons are dual-wieldable weapons that are only accurate enough for close to mid-range combat. There is no major situational difference for the use of each one. The Spiker is a better choice for dual-wielding two of the same kind of weapon. It damages shields and health effectively, which allows it to be used on any enemy. The Plasma Rifle is less-suited for being dual-wielded with a second Plasma Rifle. However, using the Plasma Rifle and a different weapon, like the SMG, that can do large amount of health damage is an effective tactic. The Plasma Rifle is also more common, which can make it a viable choice more often than the Spiker.