Zara if you're English and care about ethnicities matching in names,
Zahrah if you're Arabic (or English and just like/don't care if your kid bears an Arabic name). Also, I tend to pr.
Zara ZER-a (
Sara with a Z), while
Zahrah is ZAH-ra or ZAH-rah to me.
I personally wouldn't go with the spelling
Zahra, as I think it looks very unbalanced and incomplete.