Reactivity in SN1 reaction depends upon the stability of carbocation intermediate formed after ionization of alkyl halide. Since H3C−CH3|C|Br−CH3 will give more stable 3∘ carbocation ⎛⎜
⎜⎝CH3−CH3|C+−CH3⎞⎟
⎟⎠, therefore, it is more reactive than CH3CH2−Br which upon ionization gives 1∘ carbocation (CH3−+CH2) of lesser stability.