Macの英英辞典によると
Traditionally, compare to is used when similarities are noted in dissimilar things: shall I compare thee to a summer’s day? To compare with is to look for either differences or similarities, usually in similar things: compare the candidate’s claims with his actual performance. In practice, however, this distinction is rarely maintained.
となっていて、違う物のなかから、類似点を探す場合にはcompared to, 違う事、もしくは似ているものを議論する場合にはcompared withを使うとのこと。
(後日、例を探してみる)