If your only criteria is yield stress, not deflection, neither fatigue life, nor weight or cost and not manufacturing. Then you are solely talking about Factor of Safety. In that case, design two will handle more accidental loads than design one, under the same requirements.
Try this chart, you might find a better one elsewhere:
To solve the problem:
What is the water density and, therefore, the water mass ?
Can you now tell the combined mass ?
What do you have to know to estimate the...