Many people find the Gemini more easy to put on than the Butterfly - it is simpler and more like other familiar carriers (Bjorn, Boba, etc). If you really love the safety harness of the Butterfly and the ability to put baby in the carrier before putting the carrier on, then you might miss the Butterfly, but most people are really happy with the Gemini. For the most part, experienced babywearers prefer the Gemini and find the Butterfly a bit fussy.
The Pikkolo is very similar in functionality to the Gemini, but has the option of not using the support belt, which gives it some additional flexibility. It is a cross between a mei tai and a buckle carrier, in that when you don't use the support belt, you shed the bulky padding at the waist/hips and can wear the carrier around your waist like a mei tai.
Hope this helps!
Laurel
Owner, Carry Me Away
Mama to Jack, 5 and Maddy, 3