I like both. Unhelpful I know!
It’s strange, I like Isobel but I really don’t like Isabelle. Although I know this isn’t your chosen spelling.
If you were looking for the less popular option, Lucy would be the one, as Isobel/Isabelle/Isabella is very fashionable at the moment.
And would you going to shorten to Belle or Bella? These are also popular.
On balance, I’d go Lucy.