Over the years Magic Arcana has touched on the concepts of "functionally identical" (a card that, while not a strict reprint of another card, differs from it only in irrelevant characteristics) and "strictly better" (a card that does more than another card for the exact same cost). But as Magic grows, context changes, and so do these concepts.
Emphasis on creature type in Onslaught block, and spiritcraft in Kamigawa block, is one way that Magic has expanded its context. During the Odyssey days, the difference between Dusk Imp (Creature—Imp) and Mirage's Feral Shadow (Creature—Nightstalker) was almost nil.
However, some recent near-reprints have made quite a bit of difference. Consider:
Merfolk of the Pearl Trident -
Fugitive Wizard - Wandering Ones: Blue always seems to have a 1/1 vanilla creature for hanging around, but it matters a lot which you play, depending on whether you have Lord of Atlantis, Voidmage Prodigy, or Soul of Magma already on the board!
Unhinged further expanded that context when it introduced "name matters," "flavor text matters" and "artist matters" cards. Ekundu Griffin and Razorfoot Griffin may be functionally identical to a Callous Oppressor, but only one gets +2/+2 and trample after you cast Graphic Violence naming David A. Cherry!