1150 Single Spark for me. Slicker gearbox and less prone to problems than early 1100's (the rear slave cylinder gearbox seal requires the gearbox to be removed and opened up for the 1100 for example). Build quality; this really does depends on who(s) owned it previously. My 1150 has no corrosion, 13 years old and it looks superb.
My 5 owner 1100 had a fair bit of surface corrosion and there is no way I would try to remove nuts/bolts such as the exhaust stud nuts as they I know will happily sheer off.
The 1150 twin spark engine was the nicest I owned, but they do have issues with stick coils and those servo brakes if you opt for ABS (although if the brake fluid had been changed every year it wouldn't put me off).
The 1150 to me rides a little smoother all round, not by a huge margin, but definitely better.