Compare. To compare to is to point out or imply resemblances, between objects regarded as essentially of different order; to compare with is mainly to point out differences, between objects regarded as essentially of the same order. Thus life has been compared to a pilgrimage, to a drama, to a battle; Congress may be compared with the British Parliament. Paris has been compared to ancient Athens; it may be compared with modern London.

From: William Strunk, Jr., The Elements of Style, 1918.