I like both, so I choose Thea just because it would sound better with my last name.
Dorothea, nn Thea, changes my mind. I ordinarily like Dorothea, Dorothy, and Theodora, but they all sound heavily starched, old, and conservative next to happy, easy going Summer. I'm not in the mood for that today.
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PNL for boys: http://www.behindthename.com/pnl/116902/106592
PNL for girls: http://www.behindthename.com/pnl/116902/102654