I'm exactly 6 ft, so would the regular be...
I'm exactly 6 ft, so would the regular be "snug and warm" or cramped and awful. Inversely, would the large be too large (i.e. drafty)?
At 5'11" I'm not far behind you, and ALL my bags are longs. The extra room is mostly at the bottom on my bags, which is helpful, since I put my boots in at the bottom to dry.
The shoulder circumference for this bag in Long is only 2 more inches, so you're not gaining a significant amount of space around you.