All
"I can't think of any reason why one would use API-620 if the tank in question could be built under API-650."
I agree....Tanks designed to API 650 are designed to a very reasonable set of rules. Most flat bottomed tanks in the world are designed to 650 or the foreign equivalent.
On a side note, I would like to add that many horizontal tanks are designed to "almost ASME VIII" code.
Horizontals are designed, inspected, tested to Section VIII, but no code stamp. Design pressure is typically 10-14 psig.
IMHO, There is a hole in the API design rules about low-pressure horizontal tanks.
-MJC