I tend to play OT and rebels more often then troopers.
It depends on the strategies I like to use for each map.
Generally I use the basic soldier, Elite Rifle and Precision Pistol complement each other well.
I use Engineer a bit too for versatility and close combat boomstiktion.
I don't use marine as much in space, I preffer to just Y bomb the living bantha fodder out of all ships in view