Back on live I had a WP/Energy that was fun to play. Have to imagine that it would be good on a invul as well. I played a invul/kin on live into the twenties and an invul/stone to 50. The kin was just okay, but the stone was a beast. Loved that alt a lot. Rad is a solid choice. Does good damage. I'd lean to the rad out of your options, but energy is my second.