What exactly is the point of the KH longneck frame? I noticed that there are two 2015 19"'s on UDC (the street & trials ones), and it seems that the only difference is the frame.
Someone who owns one can probably speak for this, but I think it has to do with avoiding knee-bashes on the seatpost clamp. From an engineering perspective, this is probably the reason the longneck concept exists.
With that being said, I also think that a bigger selling reason is aesthetics - more frame, less seatpost.
for one, it’s generally a little stronger (the wider the tube, the stronger it is, and the seatpost tube has a larger diameter than the seatpost. but that is assuming they are the same material, and thickness of course) how much? marginal vs substantial depends on many factors, I’m sure there are plenty of folks who’d be willing to explain further, I try to keep engineering banter to the workplace though. : P
long neck is clearly better because it looks cooler though. : )
the shorty is 84 grams lighter, or just under a fifth of a lb.
Compare the minimum inseam specs for the two - 25" vs 30".
Unless the rider is a child or going to want the seat extra low for trials, the usual 19 or 20" unicycle frame doesn’t really seem big enough. Making the strong frame child size and extending it for adults with a long geometrically weaker seatpost might be good for mass manufacture, but doesn’t seem to make as much sense for serious use as making the frame the right size for the intended usage and user to begin with.
It also seems like a longer neck would give greater range of adjustment. If you want substantial seatpost insertion, a typical neck in the 6" range doesn’t really give you much range of adjustment if you want the seat in different places for different types of riding, or want to let someone of a different height try it.
Well, now I know that a longneck is off the Christmas list. Depending on how much squinting is involved, there’s a possibility I might not make the minimum inseam for one. 
It seems like it would all depend on crank length, saddle thickness, shoe soles, and preferred seat position, too. Unfortunately they don’t seem to give an overall length from center of the bearing to the top of the seat tube, just the length of the seat tube which is 150mm on the short one and 300mm on the long.
I own the Nimbus Equinox Street. It also has a long neck. I am 6’2". I appreciate the greater range of seat-post adjustment with this setup. I currently am practicing a combination of freestyle, SIF riding/jumping and one-footed idling/riding, and for this I keep the seat pretty high. However, I imagine that, in the near future I will be practicing seat-in jumping, riding down stairs, etc., and being able to lower the seat will be convenient. I don’t think I’d ever need the seat any lower than the minimum for this long-neck unicycle.
I didn’t notice the different inseam requirements, thanks for pointing that out to me. I’m a little on the shorter side (5’6"), but I can ride a 36" comfortably, and I believe a 30" inseam is required for one of those as well.
Unless you’re talking about a Nightfox. 