They're not the same. Love slash lets you win ties (doing 300 damage instead of the normal result where both you and the enemy take 50 damage), but you are still fucked if the enemy dodges. Super speed slash always works (for 200 damage), but goes on cooldown for one turn. So love slash is a good damage skill and a natural combo with endure (since endure leaves you without a counterattack, but love slash is effectively a counterattack skill that is also your regular quick attack), while super speed slash is a defensive skill for when you can't predict the enemy's attack.
The real overpowered skill is knockout. After successfully using it against the enemy's quick attack, the enemy can't attack next turn and any move you use will work. It turns out that this includes knockout itself, letting you stunlock bosses to death.