Yes it is a comparison. preferisco A a B (or: A piuttosto che B; A riespetto a B) But the comparison could even be implicit sometimes (just like in english): -"vuoi un caffè?" (would you like a coffee?) -"grazie, preferisco un bicchiere di vino" (Thanks, I'd prefer a glass of wine) So it's just like "to Prefer" in english, it always expresses a comparison, even if not specified in the sentence.