I chose trigger discipline to start with. Then regretted it later.
The extra accuracy is good, but the effort required to simply squeeze another shot out in VATS is way out of proportion to that benefit.
If the old F3 Grim Reaper's Sprint was in play, then I'd say go for trigger discipline, because it would gimp your character in the number of targets able to be engaged in a single VATS session, and still give you the opportunity to engage other targets in the next.
As it stands now though, you can put less rounds on target, but more accurately, but the one thing that improves throughout the game for guns type characters is accuracy, whereas rate of fire improvement really requires some effort to be put into it.
It's not really a true 5 shot to 4 shot reflection either. I think it impacts more on the time it takes in VATS for a shot to occur, although to be honest I haven't really anolysed it. One thing I'm certain of though, and that is that it doesn't matter what type of shotgun my character uses, he still only gets two shots off in a single VATS session.