IC gel superglue, hands down.
Take the coral and rock out of the water. For acros, hold the coral upside down and dry the cut edge with a paper towel. Add a dab of super glue. Dry a spot on the rock and put another dab of glue. Put the two together, and hold for a few seconds. Dunking the piece in water, or dribbling water on it actually helps the superglue to skin over faster. Within a few seconds, it sets enough that you can let go.
None of my frags came free, and I mounted probably 50 sps frags for the swap.