No offense, but i think de Dolphin slash cancel is useless, you can do the same jumping from the edge, and you will not spend your second jump >.<,
However, the dolphin slash cancel is more faster, but is punishable the same way that the ledge jumping is too.
So i think i would pick the ledge jumping because Dolphin slash cancel is HARD to perfect and HARD to execute under pressure, and they are almost the same thing.
Edit:
You can do that ledgetrick, you drop from the ledge, second jump and then aerial, The probability of getting a hit is the biggest in these 3 options and you can Lag cancel when landing ^^