They may be the same thing, hard to say without any context.
On a tank with steel roof, the top angle or the compression ring could be called a top stiffener.
On an open top tank where the wind girder is more than 2' below the top of the shell, a top angle is required on the shell, and in that case, it's not clear which would be meant by "top stiffener".
On an open top tank where there is just a wind girder, it could also be referred to as a top stiffener.
