Hi.
1.you can do reporting on both system BW and ECC.
if you develop the ECC side report it has huge impact on ECC system. and it will give detail reporting.
BW side devolved report user friendly like dragging and dropping the objects. and it will give aggregated values.
2. you can design the dash boards using ECC tables ( WEBI and xcelsius dash boards).
if you are using the BI 4.1 you can access directly ECC tables.
Thanks,
Phani.