This is an Anvil Ship, but it is still a light fighter. This class of ship relies on speed to survive and win fights. Since it's already stated to be slower than the Gladius, what other points make this ship a viable choice over it, since the Gladius got the S3 buff?

Every ship has its purpose. The Hawk is a light fighter with bounty hunting features. There are some missions for which a Hawk is suited that a Gladius isn't. Even considering their different mission profiles, however, the Hawk has an EMP device, which will play better in certain situations than a limited loadout of missiles (perhaps especially in bounty hunting), and it also has comparable shielding to and better armor protection than the Gladius. Remember too, the benefits of armor protection versus shielding against different types of weapons; although of course one prefers not to get hit, when you do get hit, sometimes you really want that armor.