Your hypothetical situation of mobs dying before Ruin can finish casting only applies to trash or adds. If you're talking about trash mobs and dungeons, Ruin II away, it doesn't matter. Otherwise, there's no difference in DPS as they both have the same recast and same potency. I also had Coil or EX Primals in mind when I typed that.

And go play SMN if you think there's no difference between Ruin and Ruin II spamming.