I have flown all of them. It's not a terribly big deal but the I2 definitely takes appreciably more time to set up than a P4P. You just plan for it and it's no big deal. As to size and portability, the P4P wins again. Once you put a P4P in a hard case with all the extra batteries and such, the size difference is not as big as one would think. However with the stock factory cases on both, the P4P is much easier to carry.
In terms of picture and video quality, there's no difference between the P4P and the X4S camera so it's a wash.
As to stability in wind and gusts, I would disagree with you. The P4P is rock solid. I have used it in 20 to 25 MPH gusty winds and the video is solid. Also a benefit of the P4P in such conditions is that you can easily hand launch and hand catch. Sure you can potentially do that with an I2 but it's nowhere near as easy. I will concede that the I2 is more stable on the ground with it's larger stance, but once in the air the P4P is easily as stable and maneuverable.
Another factor is range. The P4P and the MP have phenomenal range in all sorts of conditions. The I2 has an appreciably shorter range and I don't know why. I have tested them at the same location a the same time, and the P4P wins out on range every time.
Usage over time tells the tale for me. The P4P gets used all the time while the I2 gathers a lot of dust.
Sure there are the features where the I2 shines and the P4P cannot even begin to match up, but that's not what i use mine for. If you need the high-end cinematic cameras and lenses then the I2 is the way to go. If you are using it only with the X4S then the P4P really beats it in most every category.