Extra Line vs 6 atk

Well, best you can get in a third line is probably 3% to 6% of whatever your main stat is. Would that be greater than 6 atk? For a Level 200 Hero, I'd probably say yes. (I'm not sure, but I'm of the general understanding that 4-5 main stat = 1 weapon attack)

Either way, though, Chaos that 10 attack belt first. Worst case scenario, it loses 5 attack and becomes a 5 attack belt, which is still more attack than the 4 attack belt. Cube it and sell it as a potentialed 5 attack belt (even though it has 2 lines, people won't see that since it was cubed but not revealed). Best case scenario, it gains 5 attack and becomes a 15 attack belt, and at that point the answer seems pretty obvious.

Actually, pretty much everyone assumes that if an expensive item is potentialed but not revealed, it's 2 lines. Who potentials a 5 attack belt and doesn't even bother revealing it to see if it's three lines? And if it's three lines, who would leave it unrevealed, when a revealed one would sell for much more?

Not anymore...

Not ever, actually.

Potential doesn't truly affect your stat:attack ratio. Say you had a 4 stat:1 attack ratio before potential, and you have +50% stat, so now your ratio is 6:1. Since the potential bonus applies to both base stats and equip stats, each extra point of stat you have adds 1.5 stats instead of 1, so even though 1 attack gives you the same amount of damage as 6 stats, adding 4 stats actually gives you 6 stats, leading to the same end result.