I own both the Beco Gemini and Ergo (and an embarrassing number of other carriers). Personally, I strongly prefer the Ergo. I might be biased, because I had the Ergo first, but it ripped while I was pregnant with my second child and I wanted to try something different. I really thought I would love the Beco Gemini, since you can use it with newborns without an insert and wear the baby facing forward, plus use it in all the ways possible with an Ergo. However, I was not happy about a number of things.
1) One of the snaps was broken when it arrived and it had to be exchanged. However, I can say they have very good customer service.
2) Unlike the Ergo, which has clips that can be operated with one hand, with the Gemini, you need to use two hands to undo the clips. Since I was about 8 months pregnant when I got the carrier, I couldn't reach around myself to undo the clips without help.
3) The Ergo feels really comfortable wearing it like a backwards backpack. You can do that with the Gemini, but it feels more awkward. The Gemini feels awesome with the straps crossed in the back, but that's obviously a lot more inconvenient to put on and off.
4) Eventually, I got pretty good at undo-ing the clips and crossing the straps in the back, however I then had long straps to worry about dragging on the ground and one time I accidentally swung a strap and hit my toddler in the face.
5) Although it's possible to use it with a newborn, I didn't like using it for the first 4 months or so and strongly preferred my Moby Wrap during that time.
6) I ended up never using the forward facing feature as my daughter didn't like facing out.
7) My three year old felt really awkward in it compared to the Ergo. I could use it with him, he just seemed way too big for it.
I missed my Ergo so much that I ended up primarily using my repaired Ergo instead of the Beco starting when my daughter was about 6 or 7 months old. I still use the Ergo regularly now that she's two (2 miles today!).
There is at least one benefit I can think of for the Gemini. I used the Ergo on my back so much with my son that even when he was pretty small I felt really comfortable leaning forward, swinging him around and pulling the carrier up over him. However, my daughter strongly prefers the front. Since I don't do it as often with her, it feels a little precarious getting her on my back (more so about a year ago than now). However, since the Gemini has the straps crossed in the back, I would just put her on in the front, pull the straps off my arm, swing her around to the back, and put my arms back in. I never had to worry about dropping her.
That being said, given you have all the options in the world, I have no idea what the right decision for you is. I'm strongly considering getting a Boba, I've heard so many rave reviews about it.