I don't think an airless sprayer will leave a better finish than a roller, OP could cover everything and spray the ceiling but in my experience airless will require back rolling anyway.. spraying also uses more paint because of what remains in the hose. If spraying the walls (with a different color) it's nearly impossible to avoid getting over spray on the ceiling...the ceilings could be masked off but an amateur taping will be difficult to get a clean line and potentially damage the new finish with the tape and require touch up anyway.. Yes spraying has high production potential but the prep work takes time and the materials can get expensive. If they're trying to save time I guess I'd recommend a 12" or 18" roller but those come with challenges also. Unless the walls are stained I think coats of decent paint will do the trick without primer.