Does anyone here have experience of all three?
Yep, in the past 3-4 years or so, I've owned 4 Minigems, two GemULs, and 3 standards. At the moment I have one of each in the "new generation" build (Skyshark framing, rollbars, etc.) and one standard with Structil framing.
The Minigem holds up without a worry in very big winds and doesn't pull much at all, even in those conditions. It's quick and more prone to tip-wrapping if you're not on the ball, but once you've dialed the timings in, things work pretty well indeed. The stock Minigem is pretty pitchy and it's also probably the best kite to torpille that I've flown. The current model of standard Gems are also pretty pitchy and don't need any added ballast for snappy yoyos (whereas I fly the non-rollbar Structil version with 10g on the centre standoff). They're a nice rolly, flowing kind of kite and a lot of fun to fly with semi-improvised combinations; that's what I tried to capture in SoH3. The GemUL isn't too far removed from the standard in terms of feel, and it's a pretty robust kite considering its framing. Flat-spin stuff is generally a bit prettier than the standard Gem because you can slow it down more easily. The stock UL doesn't pitch as readily as the standard, and yoyos are best entered either using a backward-forward-backward motion or from a fade high in the window. Ballast would bring the pitch performance of the UL closer to that of the standard, but as usual, trade-offs are made.