Answer
$cyclohexyl bromide$ is the better $S_N2$ substrate.
Work Step by Step
We know, that the compound with the less hindered structure acts as the better $S_N2$ substrate.. Since $cyclohexyl bromide$ is $2^{\circ}$ and $1-bromo-1-methylcyclohexane$ is $3^{\circ}$, $cyclohexyl bromide$ is less hindered, and therefore the better $S_N2$ substrate.
