Isabel or Isabelle

What a hilarious question .
It depends, as far as I'm concerned, on what you do with the name otherwise.
Will she get extra names? Do you already have other children where the name needs to fit? What fits better with your surname.
My experience is that people always shorten a nice long name anyway. Then I think, give them the short version of the name right away. If you already know that you're always going to call her Isabel, then I'd choose that.

I think Isabelle is a bit more beautiful! Even if you always pronounce it as Isabel.

I wouldn't pronounce Isabelle as Isabel, but just go with the full Isabelle.
I think Isabel is much more beautiful! So I'd go for that.