I'd pronounce them both as REE-ə, and I'd assume that
Rheia was either a mistake or an attempt to be different.
I find
Rhea a bit uncomfortable, like other goddess names. I knew a
Venus once who was plump, stooped, elderly, had never married and always dressed in black; she was an intensely competitive Scrabble player. Didn't work. But, rather a goddess than a flightless bird.