If both are not leaking any, and you want to see which one will seal the best, find a taller container (more water weight on the clay).
Heck, if you wanted to see how the clay would perform under, say, 10', 15' or even 20' of water, get a 2" dia piece of PVC, pack 6", 12" , 18", etc. of clay in one end, drill a few holes in a cap and slip the cap on tightly (no glue). Support the cap on something porus, stand it up and fill it up with water.
