I grew up on a rhea/emu ranch, and if given the choice between the two, it will always be emus. Emus are friendly, rheas are highly aggressive, especially males during the breeding and egg seasons. Emus do have a powerful kick that rheas do not have, but the only time I've ever had to deal with it was when rounding them up and they got spooked. Rheas do not kick, they bite, and they are the pitbulls of the bird world. They do not let go until they want to let go.
If you have kids or just concerns for your own well being, avoid the rheas, go with emus. As previously stated, you get a high quality oil form them, plus a high quality meat and egg. The eggs are a beautiful deep green and are teal lightly under the surface. I've seen folks charging quite a bit at farmers markets for carved emu eggs, if I had any artistic talent, I would have done the same.