I've been doing a bit of testing involving those underbarrel grenade launchers. Turns out that the two types do make a difference, albeit a modest one. You see, the Soviet one is a bit shorter and performs a bit worse, but this allows it to be strapped to shorter barrels on your rifles. The large American one can only fit under standard size and LMG/Sniper barrels. The very shortest can only take the accuracy grips.
With the Soviet one you can more easily research sleep grenades. These are pretty much a one-hit knockout on heavy infantry, allowing you to take out clusters of them in one shot. It also reloads pretty quickly, so if you have two groups of heavy infantry you can take them out quickly. However, they do hear and see sleep grenades explode and go into caution mode, but not into high alert. Sleep grenades are also a hard counter to enemies in transport trucks: one well-placed shot and both passengers go down immediately. They do get out first, but I've had that they fell under the truck somehow, which makes it a pain if you want to extract them but want to drive around with the truck as well.
All four types of assault rifle can fit grenade launchers, as well as the AM semi-auto sniper rifle (regular and sleep ammo). This is where I slap on my sleep grenade launcher, but only partially because it's my non-lethal weapon. You see, at higher levels the long-barreled single-shot grenade launcher you can equip as your hip weapon comes with a special 2x/6x scope with range finder. This is a red targeting aid in your scope that tells you roughly where your grenade is going to land. This is incredibly useful when using your grenade launcher for precision strikes against things like radar installations. Do note that this only works in 2x zoom: at 6x the targeting aid drops off of the scope almost immediately if you aim upwards. I prefer to use the destructive grenades at a larger range while using the sleep grenades at short range when it's easier to guess where the shot is going to land. Thing is, while a 6x scope is just fine for an assault rifle when going prone, it just won't cut it for a sniper rifle. So the 6x scope is ideal if you want both the ability to engage enemies at a good distance yet want to use your underbarrel grenade launcher as well.
However, when you use lots of gas-based grenades (smoke and sleep) enemies start wearing gas masks. This is actually quite a useful thing: from what I've seen helmets, gas masks and night vision goggles are all mutually exclusive. Sure, you can't knock them out with sleep gas anymore (100% immune), but they're easy pickings for both you and Quiet but they won't see you walking around as if you're in broad daylight. Additionally, heavy infantry is defenseless against sleep gas and you can easily knock out wandering puppets with two sleep grenades.
Of course, grenade launchers tend to heavily increase the cost of your weapons but you're essentially able to carry another weapon, which is quite useful given the limited equipment system.
Furthermore, I've noticed that underbarrel weapons of all kinds (grenade launchers, shotguns and the pistol(?)) restrict the types of magazines you can put into your guns. The 7.62mm has 75 and 50 round drums while the 5.56mm has a 100 round double drum, but all of those cannot be equipped with an underslung weapon (and unequip these magazines upon equipping them). This limits you to either of three magazine types: the basic dirty ones, the gaudy orange ones and the tiny magazines carrying the larger rounds, limiting you to a total of 60-something shots.
I also noticed that the non-lethal weapons that stun can knock an enemy out with a single headshot, but enemies around them will react as if their comrade was hit by a lethal round, as opposed to the surprise you get with a sleep round.