The creature with Flash version can target itself, too, and also avoids the feel-bad of being unable to cast it on an empty board when your opponent is at 1 and you just need any creature. So they do differ in somewhat significant ways. Amusingly, the creature strikes me as a borderline uncommon while the instant is clearly common.
Zealous Guardian is a 1/1 flash for 1 with no abilities at common, so a 1/1 flash for 1 with a combat trick attached does seem uncommon, eh? (See also: Briarhorn > Briarpack Alpha.)
Especially if it can target itself.
Another difference I just realized: the instant is better with Heroic. Helps that the flavor is a perfect fit for Theros.