I like Isabella better for a fn, Isabelle for a mn. My favorite would be Elizabeth, nn Libby. If you want a nn for Isabella I would go with Bella. Good luck!
I love Elizabeth. Its so sweet and classic. So that's my vote. I prefer Isabella over Isabelle. I just think its prettier. I like the nn Bella a lot. You could always use Issy, but I prefer Bella.
I like Elizabeth best, because of all the nn possibilities. They're kinda limited w/ Isabelle/a. Of the other two, I prefer Isabelle over Isabella.
GL