Ok first we need to know how old your tank is and all the information. Equip, LR
The reason I ask is because one of these fish may be a bad choice and the other is absolutely a bad choice for a new tank.
Damsels can become aggressive although people do keep them. They will eat Mysid Shrimp, meaty foods, and probably flakes/pellets.
The Scooter Blenny is a difficult fish to keep. Many sites have misleading info. Its main food source is pods
as with its cousin, the mandarin. It is a dragonet and not a blenny and its chance of not starving to death in a new tank are not too good. A well established refigium or a seperate pod culturing setup is needed. Some and only some will accept prepared foods but most are to big for them. Please return this fish if your tank is new.
You can ask for any fish you are thinking of getting and most people can help.