Heavy Laser vs Heavy Cannon
The (plain vanilla) Heavy Laser, because of its position in the Laser weapon hierarchy, is most often compared against the Heavy Plasma. Refer to the Heavy Laser vs Heavy Plasma section for a comparison.
For those unfamiliar with the term plain vanilla, that just means plain. Boring. Dull. In this case, this refers to an unmodified Heavy Laser and not, in particular, the "enhanced" Heavy Laser introduced with XComutil's optional enhanced laser settings.
It's a decent weapon but more often than not, the Heavy Laser is quickly discarded because it doesn't share the same level of performance that the Heavy Plasma can achieve. Despite this, there are a still a large number of players that are very loyal to the heavy laser, for any number of reasons.
After careful consideration, it was determined by this author that comparing the Heavy Laser against the Heavy Plasma is perhaps not the right way to gauge the Heavy Laser's effectiveness. They are completely different classes of weapons. Instead, the more logical comparison to the Heavy Plasma belongs to the Heavy Cannon, which is also a very decent weapon in its own right.
Stat Comparison
(insert side-by-side table for stat comparison here)
For general Assault
Snap shots are perhaps the most important and time efficient shot modes available in the game general assault weaponry. Notice how the firings costs for both weapons are identical: 33%. Where the weapons differ from each other is in their accuracy and shot damage. The Heavy Cannon is the clear winner in accuracy, being 10% more accurate. However, the Heavy Laser inches out by having much more stopping power in its single attack. The Heavy Cannon also has the advantage of area-effect rounds, which allow it to perform better against weaker targets.
In a sense, this puts them on fairly equal footing against standard enemies. You could even say that they would compliment each other very well by using the Heavy Laser to replace HC-AP rounds, allowing for better finesse and less collateral damage, while using the Heavy Cannon strictly for HE and Incendiary attacks. If accuracy is the issue, switch to the Heavy Cannon.
Against tougher enemies like Mutons and Sectopods, the Heavy Laser may prove to be more efficient.
For Sniping
Though neither weapon make for the best sniping weapons, many commanders swear by both the Heavy Laser and Heavy Cannon and can be very effective with them when employed correctly.
This section compare their aimed mode fire. For snap-sniping, refer to the previous section.
Referring to the stat comparison, the Heavy Cannon is much more accurate but considerably more time consuming in its follow through. The Heavy Laser is less accurate, but also less time consuming. As both costs only allow one attack per turn, and because snipers rarely ever have a need to move from their positions, the speed/accuracy trade off is not very meaningful. This leaves the Heavy Cannon as the more accurate sniping weapon.
When single-shot power and ammo restrictions are considered, the Heavy Laser leaves the Heavy Cannon far behind.
Therefore, if ammo and damage is not a concern, the Heavy Cannon is the better sniper's weapon. If ammo and damage is a concern, then the Heavy Laser is the best of the two.