What is a person’s height that is the dividing line between choosing a twin mattress vs a twin xl?
Good question. I first ran into the XL when I went off to college in ‘65. It was a California school and people were tall there. I was 6’ 1" and average height in my freshman class. The dorms all had XL beds and from my experience I would guess anyone over 5’ 10" would appreciate the extra length when sleeping on stomach or back. I have no idea what truly tall people do. I guess they sleep on their sides. 
Hi powerdog,
The most common “recommendation” you will see about a twin vs twin XL is that anyone over 5’ 5" should choose the twin XL.
I think though that your own personal experience and sleeping style (as LEW mentioned) would be more of a factor though than any “generic” suggestions that don’t reflect any specific persons actual experience. I am 6’ 5.5" tall and if I was to believe “generic” suggestions that indicate that you need an extra 5" beyond your head and feet then I would need a mattress that is 87" which is three inches longer than a Cal King and I sleep quite comfortably on a queen (which is only about 2" -3" longer than I am). I don’t think that most people sleep fully extended to their full height and exactly parallel to the mattress so I would trust your own experience in real life rather than any “official suggestions”.
Phoenix
The XL would be better then, even though I’m only 5’8". If the top sheet is tucked in tightly, and I happen to be on my back, I often find my feet are compressed.
Yeah, if there is any question go long. For years my wife and I slept comfortably on a full size mattress, same length as a reg. single. But I built the bed without a standard headboard. Instead I built it with a flat board at the top of the bed that was a little above mattress height. It allowed me to put my pillow over the end of the mattress and not be confined by the headboard. It also replaced bedside tables. I could put a glass of water, books, etc on my end table. It worked pretty well until my wife decided that our house should look like grownups lived there. :lol:
Our dog often invites herself to sleep at the end of the bed. The extra inches can be her space.
There ya go! Extra room for the dog. We recently adopted a second cat and now the two of them take up a third of the width of our king. 
The difference between a twin and a twin xl is only about 5 inches. A regular twin is about 39 in × 75 in while a twin xl is 39 in × 80in
So really, you’re looking at a 6.25 feet for a twin and 6.7 feet for an xl. Figure you need to remove a couple of inches for pillows and comfort, etc (no one wants to sleep with their head against the wall.)
So, I would say anyone over…6’1 would be more comfortable on an XL