Gilgamesh CE vs Sumo

Sump increases ALL of your card's attack. Buster effectiveness Increases damage of ONLY buster cards, so IMO sumo's better damage wise

They're the same but stacking both is better than the same amount of one(ie. 15% buster and 15% attack is better than 30% attack) due to buffs stacking multiplicatively. To expand a bit there are 4 catagories of buffs - attack up, card performance up, other, and NP special attack(think Robin Hood, Gil, Medb, etc) which each stack multiplicatively while buffs within each catagory are additive so for maximum damage you want to have at least one from each catagory.

Edit:Thus to answer the original question Sumo will do more damage as Raikou already has a buster up.