The interesting thing about emu is that as you carve you expose different layers of color, dark green to a blue -white. This egg is very popular for etching. Goose is a nice reliable carving egg, white to ivory in color. It has a little bit of texture to the shell and makes a nice smaller carving egg. It is perhaps a little more durable than duck egg, but very impressive when carved and filigreed.
Duck is an easy carving egg of white color with a smooth almost porcelain quality to the shell.
They range in size from chicken size to almost as large as normal goose egg. Chicken eggs can be done. However, the normal supermarket egg is unreliable and cracks easily. On the whole, I like duck better for most applications. The little guys like pigeon and canary can be done, but they are a custom order only, murderous to produce, enough to make me seriously cross eyed.