DB2 concat vs double pipe

From Information Center:

The CONCAT function is identical to the CONCAT operator. For more information, see With the concatenation operator.

So, the definitive answer is: they're the same. However, IBM does have this to say:

Vertical bars (or the characters that must be used in place of vertical bars in some countries) can cause parsing errors in statements passed from one DBMS to another. The problem occurs if the statement undergoes character conversion with certain combinations of source and target CCSIDs. Thus, CONCAT is the preferable concatenation operator.