The biggest difference between the Gemini and the Ergo is the outward-facing front-carry option that the Gemini has, so it's more like the Bjorn but more ergonomically designed. The babywearing guru I tested it with called it a "gateway carrier."
The second big difference is the lack of a need for an insert to use the Gemini with a newborn - it has snug-cords that can adjust the carrier pouch so it can hold a newborn.
The last big difference with the Gemini is its flip-up hood vs. the pull-out sleep hood with the Ergo. If you use a hat and the flip-up hood, your baby should be perfectly happy this summer and next summer the back carry will be fine to use. (In between, the Gemini will fit nicely under a winter coat in a front-carry!)
I have a black Ergo with a camel colored lining. I used my Ergo in the summer with my DD, who is now 11 months. The black fabric didn't phase me; the infant insert, on the other hand, did. It was simply too big & bulky to wear comfortably in hot, humid weather! (So I used my woven wrap carrier on those days instead.)
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle