I really like both. I agree with others that Saphira looks more soft and feminine. Spoken I think Zaphira sounds great, although written I keep seeing the word 'Zap". Sorry - JMO! Another reason why I'd go for Saphira.
I have a 3 syllable limit with names - so Ariana would have to become Ariane or Arielle to compete with Zaphira/Saphira for me.

Good luck deciding. Neither are common names, but Zaphira definitely more unqiue.