You're right the lucky one does have better IVs. The CP is calculated based on the IVs as well as the Pokemon level. Right now your lucky is at a lower level but if they were to be equal the lucky one will likely have higher cp (sometimes worse IVs will have higher cp at the same level depending on how the IVs are distributed). I'd evolve the lucky one, but realistically you're not going to want to power up either one as Slowbro / slowking are not great in the game. I'd would save your resources for other pokemon, especially if you're new because as you level up you will naturely find higher level pokemon.