They are two different products, although the SQL at the DML (insert, update, delete, select) is almost 100% the same (there is a DB2 manual which compares SQL compatibility on the IBM Website where the manuals for DB2 are listed).
Of all the DB2 products (LUW, z/OS, VM, iSeries), iSeries is the most different from all the others because of its close integration with the iSeries OS.