Laser Rifle vs Heavy Plasma
In general these two weapons are very similar in their characteristics, apart from the base damage of the Heavy Plasma being nearly double that of the Laser Rifle. All else being roughly equal, this key fact means that the Heavy Plasma has twice the firepower and in general will kill any given target in half the time (on average).
The noted exception is when fighting Sectopods. Against these plasma-resistant, laser-vulnerable war machines the base damage levels nearly equalise (90 for the laser, 92 for the plasma weapon). This then cuts the Laser Rifle's firepower disadvantage to 25-30% in auto fire (the most effective mode), a wafer-thin advantage in snap fire (including all human reaction fire), and a significant advantage in aimed fire (though unfortunately this is the least effective fire mode). So even versus Sectopods, there is no real firepower advantage to the Laser Rifle. (Actually it is the Heavy Laser that edges out the Heavy Plasma against this particular enemy, with 3x the firepower of the Laser Rifle and 4x the firepower of the Heavy Plasma.)
Ammunition is a difference but probably not a significant one. A Heavy Plasma clip is good for 5 rounds of fully sustained fire (2 burst and one snap shot using 95% of TUs). It is rare to have the opportunity to fire this much in a mission. Nor is it usually necessary to use weight, inventory space or Transport space carrying extra clips, as they can easily be scavenged off enemy casualties. But again, fighting Sectopods (especially without heavy weapons) is one situation where the Laser Rifles ever-lasting ammunition might make a difference, since it takes an average of 24 hits from a Heavy Plasma to kill a Sectopod through its front armour. Even with a firing accuracy of 100, that would represent 16 bursts - 48 rounds - fired on auto.
Clearly the Heavy Plasma is the more expensive weapon system to own and operate. Heavy Plasmas that are used on the battlefield, rather than sold, have an opportunity cost of $171,600, vs $36,900 for Laser Rifles - 4 times the cost. The operating (ammo) cost of Laser weapons is zero, whereas the cost of equipping troops with Heavy Plasma is $9,590 per mission per soldier who uses his or her weapon - assuming captured clips are available. If they have to be manufactured, the costs rise to around $24,000 per clip, not counting loss of time,opportunity cost of more profitable production (such as Laser Cannon production), and the fact that they use Elerium. Remember, scavenged clips are not "free" if you put them in the field - you are losing $9,590 for every clip you don't sell. For all these reasons, if money is in short supply (for example if you avoid using any of the Economics exploits), it may make sense to equip at least some of your troops with Laser Rifles instead of Heavy Plasmas.
From an Experience gain point of view the Laser Rifle has a slight advantage (this is generally true of weaker, more rapid-firing weapons). In reaction fire and in 100% concentrated aimed fire the Laser Rifle on average will generate more hits per turn, giving more experience. Even on Auto the advantage to the Heavy Plasma is less than 10%. This is another argument that suggests arming some troops with Laser Rifles. (In fact, the Laser Rifle is probably a better training weapon that the Laser Pistol, since the balance of firepower and experience is superior in Snap mode and about equal on auto mode - but rookies with Laser Pistols on auto can be a danger to their own side).