Frogspawn, Hammer coral, and galaxea and many others are all part of the "euphilia" family. They all have nematicysts which can sting other corals. The main difference between each one is if it is branching, non branching, as well as the different shapes of the polyps.
When placing either one, you need to take into consideration having a space between them and other corals. The members or the family euphilia are quite beautiful, and grow fast. You had better have a nice sized tank, or know how to frag them.
I would get both the frogspawn and the hammer, and place them at the two ends of the tank. Don't be surprised if they do not do well in your aquarium. They are kind of picky and selective, somewhat the same as xenia in that way.
Good luck!