I think both are beautiful and creative names, so I've put together some pros and cons/questions for you to look at that might help you decide (if you haven't already):
-Everyone knows that when they got in trouble as a kid, they're mom called them by their first and middle names together. Which one will have a better ring to it? ("ELla diANE!" or "keIRA eilEEN")
-Ella is an adorable name for a little girl, but will it still be as nice when she gets older (I think it could go either way so it's really what you think)?
-Will everyone come up with an obnoxious nickname for one of them? A nice nickname for one of them?
-Is one of them too similar sounding to one of your other children? (e.g. you might not want something like Bob and Rob or Casey and Lacey--unless you like that sort of thing :))
All in all, it's totally your decision, so choose whichever one you like better! I personally think both of them are pretty and unique (;