Different kites altogether - so to speak.
Nirvana is a precise, strong pulling delta. Well capable of all the modern technical tricks but does require an assertive approach to flying it. THe kite needs to be 'dominated' (for want of a better word) into tricking - Not that tricks are difficult on it - just require a very 'definite' input. Think flying from the shoulders, rather than from the wrists.
Gemini is a much more playful kite - Nowhere near as precise as the N, but then it's not meant to be. Easier to trick in general than the Nirvana, but with a slightly different trick repertoire - for instance the N is much more capable on it's back with turtle based tricks compared to the Gemini, but I've always found the Gemini easier to learn and refine tricks on.
Both are excellent kites - Among the top of their respective fields, but as I said - they are different to one another.
I've recently sold my Nirvana, but I'd be reasonably sure to have another in the bag eventually. As with Desciple - complete Gemini series in the bag is always worth having