Get one as large as you think you can handle to take care of. You will never have enough room because you always want to grow more and different things.

Your choice seems easy if it were up to me. Although the 16' height seems higher than normal (my 30' GH is 13.5' at the peak) you will find that the air buffer is better in the larger space even though you will need more heat input when you need to keep it warm at night.

The other big issue is the height near the sides. My 20' is higher than the quonset you mentioned(10.5 at peak) but at about 3' from each side the arches are only about 5' high, limiting the crops I can grow there. A quonset hoophouse is worse than that.

And one other concern is that you may even someday want to construct a canopy within the structure to gain a few more weeks growing season. The higher, wider structure simplifies that task.