Generally speaking, the Spectator's ability is far superior.
The Spectator can create food for other people
A Spectator can create a day's worth of food and water every six seconds. A Banshee can't do that, it just doesn't need to eat. A Spectator could use that ability to keep pets or companions, function as the food source for a moderately sized army (a spectator can feed 4800 people with 8 hours of work), or cut down on overhead costs for its food stall.
The Spectator can use food and drink for purposes other than eating or drinking
A Spectator can drown an enemy that can neither swim nor escape a pit with water, or enable an ally to cross a pit with water or food or both. A Spectator can block up a door or passageway with a wall of food. A Spectator can make little statuettes out of food and then play with them. A Spectator can use food or water or both to trigger or bypass weight sensors such as pressure plates. A spectator can create a pile of food which then rots and attracts Otyughs and repels humanoids. A Spectator can create a basket of bone-in wings and then have a pixie minion use one as an improvised club. If imprisioned in a cave with a friendly wizard stripped of their stuff, the Spectator can provide the missing component for the spell Flesh to Stone (It's water. Lime and dirt are usually omnipresent in a cave).
The Spectator can starve to death; its ability can be disabled
If the Spectator wants to take its own life and has no other means of doing so, it can dehydrate to death and/or starve. This can backfire, though, as other creatures can potentially kill a Spectator by starving it to death or preventing it from drinking enough water, for example by placing it in an Antimagic Field.