Both are very stable and both can be very fast. The supernova feels lighter on the string but again, both can easily play fast. The supernova is wider than the genesis so that is something to consider. Genesis is cheaper. Supernova can’t grind to save your life (I’m only talking about the 7075 aluminium supernova). The only diffrence between the supernova (7075) and the supernova lite was the metal so the 2012 supernovas are basically supernova lites.
I say (regarding 7075 supernova) that it depends on your play style. The supernova is wider so if you get very technical then you may want to swing towards the genesis. Both have amazing spin times and are both very stable and fast (supernova feels lighter though). I have had 4 supernovas and 2 genesis yoyos. I suppose if I had to choose I would say genesis since I have a soft spot for them being my second full metal yoyo, hehe.