Answer
2-methyl-1-iodopropane 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 2-methyl-1-iodopropane is $1^{\circ}$ and $tert-$butyl iodide is $3^{\circ}$, 2-methyl-1-iodopropane is less hindered, and therefore the better $S_N2$ substrate.
