I am not sure SUM is just syntactic sugar for simple sums of a colum, or that SUM ( 'Table'[Column] ) is shorthand for (and will be translated to by the engine at query time) SUMX ( 'Table' , 'Table'[Column] ).

In my experience is a performance difference:

see https://maxpowerbi.pro/the-dax-summary-pattern/

It's also what Marco Russo and Alberto Ferrari recommend https://www.daxpatterns.com/currency-conversion/