SMAW has more versatility in being able to free fire OR lock onto aerial killstreaks, but with the trade-off of just 1 rocket and is also pretty weak against vehicles besides UAV stuff. (Can't even take down an Attack Helicopter, IIRC, unless it's already damaged somehow.) Of course, the usual downside also being that if you lock onto stuff with flares, it can be diverted.
RPG has 2 rockets and more damage, but at the cost of slightly less accuracy (as said, it can sometimes be pretty accurate, but be HORRIBLY inaccurate at other times, it's pretty random like that), but also has the advantage of not worrying about flares if you try to fire at the Pave Low, Osprey, and so on as it's a free fire rocket and not lock-on.
So really, I think it primarily comes down to personal preference followed by the map type in terms of Launcher choice. Like on Resistance, I tend to go for the RPG. Firstly, because people love to camp in the middle house, so the blast radius and damage from 2 RPG rockets will usually be able to kill or flush out the campers in there, or you can use it to blow up vehicles and if the rocket explosion itself doesn't get them, then the explosion(s) from the vehicle(s) will.
