It all depends on you. I don't find any of these options comfortable, many people do though. I will say the Gemini is very small bodied and won't last. The Soliel and Ergo are very similar. So really it all depends on fit. I'd try to find a Babywearing International near you to try them on.
I've used the Ergo in the past (once DD hit about 15lbs, it hurt my back), and now use a Soleil almost exclusively, because I like that I can cross the straps, and adjust how close in she is. I also lile it for the back carry. However, I think it's a really personal preference, so I second PP re: trying them on.
If you are under 5'2, you may want to rule out the Ergo. As a babywearing educator, I get that feedback from moms regularly and I find it true for myself, too. I like the Gemini and I do think it has a long-life. Baby won't have fabric spread knee-to-knee as long with the Gemini as with the other options, but the knee-to-knee thing is more important for younger babies anyway, since they lack muscle tone. I prefer woven wraps to other kinds of carriers, but if I must use an SSC with my kids in a pinch (and for teaching SSC use), I use a Beco Gemini.
When you look at a stock photo, you'll see the fabric folded at the top of the body. That unfolds to be a head support for a newborn or a nursing/sleep half-cover. I found this via Google images:
Re: Beco soleil,Gemini or Ergo baby??