The mechanisms of reactions $(i)$ and $(ii)$ are:$(i)$ Isobutyl bromide $+{\text{C}}_2{\text{H}}_5\text{OH}\to$ Ether$(ii)$ Isobutyl bromide $+{\text{C}}_2{\text{H}}_5{\text{O}}^{-}\to$ Ether

Primary halides (like isobutyl bromide, though slightly hindered) generally prefer $S_{N}2$. With a weak nucleophile/protic solvent like ethanol $(i)$, it proceeds via $S_{N}2$ (or $S_{N}1$ if branching is extreme, but usually $S_{N}2$). With a strong nucleophile like ethoxide $(ii)$, $S_{N}2$ is clearly favoured.