From what I can tell, the main differences is that periodic madness adds building to do more chemistry, whereas I don't because I'm not a good enough 3D modeller. I think also the purity system in Elemental is a difference, but I think the main difference is periodic madness is probably better, because it's been around for a few years. Also the bio modules look really cool, I wouldn't even know where to start programming something like that!
I think the main differences is that periodic madness probably has more polish and is probably better (definitely at adding more elements). I would recommend trying both of them because we seem to be tackling it differently.
Hope that answers your question :)