I use both. I use SimLauncher to start/create a flight. It's easier for the testing I do so I am not creating a bunch of flightplans - mostly hand-flying, testing airports, doing circuits. But I use Plan-G mostly as a follow map. I suppose if I was creating flightplans I would go with Plan-G since you cannot create a plan with SL, only load them.
FYI - I use the older version of Plan-G (one version back from current), with Prepar3D on a separate laptop. Works great.
Clutch