Sentence Correction

MGMAT guide 8 says GMAT ignores the traditional distinction between COMPARED TO (emphasizing similarities) and COMPARED WITH (emphasizing differences), where as, the flashcards from BTG say COMPARED TO is used for UNLIKE things to show similarities and COMPARED WITH is used for LIKE things to show either differences or similarities.

Which of the two should I go with if I see a question on the test?

As far as I know, this issue is not tested exclusively. Meaning, if this comes up in an SC question, there will be OTHER errors in the answer choices to allow you to identify the correct choice.

I cannot recall ever seeing an "official" question that focused exclusively on this error. So I don't believe the GMAT really does test any distinction here.

That being said, typically "with" implies a comparison between similar things, and "to" implies a comparison between dissimilar things.