I believe ISDI was a terror, planetary bombardment weapon, so had alot of slow to fire inaccurate heavy turbolasers in one direction. The ISDII was developed to be more resilient due to the growing rebel threat, with stronger ship to ship capabilities, such as more armour and faster weapons, which would be better against star fighter rushes. The ISDI will still hurt a cruiser or shipyard, but the ISDII is designed to fight of smaller fsater ships.