you have to look at what's different between the two systems and assess the strengths and weaknesses of those differences and how that changes the rate of the reaction (do you know what kind of reaction you're getting? SN1/SN2/E1/E2?). Possible differences: nucleophile, nature of electrophile (methyl/primary/secondary/tertiary/beta-branched..), solvent, and leaving group.
System 1: 2-chlorobutane, NaI, acetone
System 2: 2-bromobutane, NaI, acetone