Let's just say that you HAVE to become Sin, and yes, you have to try to live up to what Sin is. The only upside is that you get to choose how Sin looks and what it can do. So...how are you Sin?!
Personally, I would love to be a giant laser cat. The fur coat I have is made of pure energy beams. I can do a nifty heat vision thing with my eyes, and I can shoot my claws out as sin spawn! The funny thing is that my meow would still sound like a regular, cute cat.