Out of and which is more reactive towards reaction and why?

The order of stability of carbocation is maximum for tertiary, followed by secondary, primary and methyl in the same order.

Since the rate determining step in reaction is the formation of carbocation, and because 1-chloro-1-methylpropane would form secondary carbocation and 1-chloro-2-methypropane will form primary carbocation, which is less stable than secondary carbocation, the reactivity towards reaction would be higher for 1-chloro-1-methylpropane.