I'm actually thinking I'd probably have different rules for different spells:
* Faerie Fire: explicitly says it cancels the benefits of invisibility, so the creature doesn't receive advantage, attackers don't get disadvantage
* See Invisibility: allows the attacker to see the target (so no "you attacked thin air and hit nothing"), but target is still difficult to see, so advantage to creature, disadvantage to attacker
* Truesight: It's a sixth level spell, so it does what See Invisibility doesn't. No Advantage/Disadvantage