Volcarona is one of my favorite bug types, if not the. Genesect really can't be compared because whilst legendary, in terms of competitive battling (and even just a regular one really) Volcarona is a full out beast.
In a battle between the two, I see the obvious winner being Volcarona. Well first to begin with they're both bug types, but Volcarona has the advantage. x4 Fire weakness on Genesect couldn't be any more easier for Volcarona to sweep. Of course it all varies on what drive you give Genesect, but you know. Volcarona doesn't need some kind of CD drive to be great.
Plus Volcarona also has a pre-evolution and it's adorable. I think it has the win/win situation here, since its design is also amazing. To be honest when Genesect was first released I thought it was bland, and even now too. It rubbed off on me a little but first of all, the shade of purple isn't the best and second, it has no appeal to me as Volcarona does. The shades of red, orange, blue, etc. they are just really appealing to the eye.
So in the end based on everything that could be told about these two Pokémon, Volcarona gets my vote. :B