A single XR15 G5 over a 20" cube isn't enough to do SPS or high light corals except in the center and up off the bottom of the tank, and even then it would be running at or near 100% for a reasonably long photoperiod. The G6 isn't more powerful, but the light is distributed more evenly, so the area where a high light coral would be really happy is probably even smaller in the center. I eventually moved to an XR30 over that tank and run it around 65% for mixed reef with a bunch of SPS.
It's plenty, and probably is a great choice for softies and LPS, and plenty of photosynthetic stuff will do well under it, but I would basically consider acros and clams out of the question unless you have one or two spots high on the rockwork in the center if you opt for a single XR15.