I love both.
Claire has more of a sophisticated edge.
Clara is softer and warmer in comparison. As a first name, I would be more inclined to use
Clara since it isn't as popular as
Claire.
As far as spelling goes, I prefer
Claire to
Clare. I don't really like the look of
Clare. It doesn't look feminine to me.
I pronounce
Clara KLER-ə (like
Claire with an additional syllable) and
Klara KLAH-ra. I prefer
Clara. It doesn't sound as harsh to me, but I like both
Clara and
Klara.