The vanilla heavy splatling is definitely more of a defensive weapon with range, splash wall, and inkstrike. You want to stay in the back, cover turf, disrupt enemies, and fire off inkstrikes.
The deco version is more offensive minded, with point sensors to tag enemies and the kraken for getting right into the action. This one can be both offensive and defensive, however, because you still have a lot of range and you have the option of using the kraken as an "oh crap" button on defense.