I was watching some X-COM YouTube videos by falkor95 the other day. (I love his british "color commentary" while his partner in crime named "Matthew" runs the controls. It's too bad the background music is so high, I would have liked it 100% more with just the commentary). Anyhow, he was on a horrific terror mission with Ethereals and Sectopods and the squad was mainly equipped with Plasma Rifles. The result wasn't good because the Plasma Rifle bolts didn't seem to phase these creatures.
Well, I ran some numbers on this just to see what is up. A Sectopod has the following stats:
Experienced-Superhuman Front Armor: 145 Left/Right Armor: 130 Rear Armor: 100 Under Armor: 90 Beginner Front Armor: 72 Left/Right Armor: 65 Rear Armor: 50 Under Armor: 45A Plasma Rifle has a listed damage of 80 (range: 0 - 160). However, the Sectopod is 80% susceptible to plasma damage and hence the range is lowered to 0 - 128. Comparing the max damage to the Sectopods armor we can clearly see that the Plasma Rifle is 100% ineffective against the front and side plates. Gee, no wonder he had so many problems. Even if you managed to circle around and shoot at the rear plates, you have a 22% chance of breaching the armor. To put it another way, 78% of all attempts will do 0 damage if shot up the arse. Not good.
On beginner, the odds are a bit better.
Front: 43% breach
Side: 49% breach
Rear: 60% breach
Still, even these odds are nothing to sneeze at.
Now let's look at the Laser Rifle. It has a listed damage of 60 and its range is 0 - 120. That's 20 points less than the Plasma Rifle. However, Sectopods are 150% susceptible to laser damage. Its range is now boosted to 0 - 180 (52 points more damage than the Plasma Rifle). This increase is enough to do damage against a superhuman Sectopod's front plates.
Front: 19% breach
Side: 28% breach
Rear: 49% breach
I'd much rather be toting the Laser Rifle when going up against Sectopods. It's cheaper to manufacture, easier to come by and doesn't chew up ammo either.
The break-even point between the Plasma Rifle and an equivalent Laser is 43 (for a Sectopod). That makes the lowly Laser Pistol the Plasma Rifles equal (at least in terms of damage). By the way, the Laser Rifle is approximately equal to the Heavy Plasma against the Sectopod. (It's a little bit less, but right in the range).
So as you can clearly see, Lasers (any type) are better than Plasma against Sectopods.