Answer
1-iodobutane 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 1-iodobutane is $1^{\circ}$ and 2-iodopropane is $2^{\circ}$,1-iodobutane is less hindered, and therefore the better $S_N2$ substrate.
