The way I see this going is that Sans would open with a move that isn't an instant kill, since he's in character and would want to figure out his opponent first. He'd take damage from the attack being reflected, but my question is would the thorns potion reflect only the initial damage of the bone/gaster blaster, or will the potion reflect Karmic Retribution too? Terrarian has way higher durability so any move Sans uses that can do significant damage will be reflected back on him, and he is nowhere near as durable.
Ultimately, I think Terrarian wins here. They have higher AP, durability and a far more versatile arsenal, so unless Sans goes all out from the start (which he doesn't since he'd in character) I don't think he can win. If nothing more comes of this thread, count my vote as Terrarian, but I am happy to discuss this because its not a super clear cut victory.