Respuesta
$\text{S}_\text{N}1$ is unimolecular (two steps via planar carbocation, leading to racemization); $\text{S}_\text{N}2$ is bimolecular (one concerted step with backside attack, leading to Walden inversion).
Explicación
$\text{S}_\text{N}1$ is favored by tertiary substrates and polar protic solvents, whereas $\text{S}_\text{N}2$ is favored by primary substrates and polar aprotic solvents.