I am about about 6 foot and a half inch...
I am about about 6 foot and a half inch tall 188 pounds. Would someone this size usually prefer the regular or the long?
I'm the same height as you... but much much skinnier (135 lbs.) I always a buy a long sleeping bag. Yes, you could fit into a regular and cut some weight but then you'd be compressing the loft towards the feet and would be quite cold (you need the loft for warmth!). Also, I doubt you sleep with your feet pointing straight up, which is also another reason to buy the long.
i agree with sam that you should buy the long. however, you'll have a little too much extra room by your feet and they may feel a little more cool than you would prefer. if you shove some extra clothing in the footbed, it should help.