I was going to say I like Evie as a potential nickname for Eva, but I see that’s not the question. Honestly, I like them both. Evie is sweet and a bit younger and fresher sounding to me. Eva is pretty and more mature. If I had to pick just one as a nickname I’d go for Evie, assuming her given name could be used if she wanted a more mature professional name as an adult.
As a nn? Evie. But I LOVE Eva. Which is probably why I couldn’t commit to Evangeline when I tried–I just love Eva and Evie too much! And as Eva’s a legitimate name on its own, it seems sort of odd as a nn. Eva nn Evie just works better for me. But Evangeline nn Evie or Evangeline nn Eva are both gorgeous, and I don’t know which I’d be more excited to meet. As a nn, I prefer Evie, but in general, I like Eva more than I like Evie. Although Evie is adorable, and I’d be tickled-pink to meet one.