This answer is not correct but your thinking was logical in that both compounds are primary halides and both would have about the same

This answer is not correct but your thinking was logical in that both compounds are primary halides and both would have about the same steric effect when reacting by the S_N2 mechanism.

Even though the S_N2 mechanism does not proceed through an intermediate, the transition state will have a great deal of carbocation character and is actually stabilized in the same ways that carbocations are stabilized.