Hi,
If you want to compare these two, I definistly recommand you to use Open SQL anywhere in your ABAP program, because it will improve your performance of your program by using system cash of SAP system, which Native SQL can not achive. Fetch array is very comman in Oper SQL which used for Extract Data from database such as SELECT * FROM MARA INTO TABLE itab, for update and modify data to database such as MODIFY dbtab FROM TABLE itab.
If native sql process rows of data, it will process one by one, its network is very costly to compare with Open SQL.