To a certain extent, the roof type is also determined by government regulations.
Generally, a light product (gasoline, for example) is going to require either an internal or external floating roof or some type of inerting/ vapor recovery system. The exception being smaller tanks.
Generally, a heavy product (diesel, fuel oil) can be stored in a fixed-roof tank wtih no floater.
For a small tank, a self-supportign roof is easiest, for a large tank, a structuraly supported cone roof.