I’m really hesitant about what to buy and i want to know how much difference dus weight really make? can you really tell a difference? can you still do the same things?
Not much. You gain very little hop hieght. Yo don’t need a light uni to hop high you just need to gain the comfort and strength with a heavier uni and you’ll be able to hop just as high with a dx as a Kh. I’ve tested a kh and own a dx and I noticed no difference. And anymore I preffer a heavyish uni because of its rolling stability for skinnies and stuff.
The weight might not matter too much for trials but if you are going to do flatland/street stuff with unispins and tricks like that then it is so nice to have a light uni. For fast flip tricks or fast unispins it is really hard to whip it around with a heavy uni.
for muni i feel better with my heavy ass steel GB4 than on others i’ve ridden. not that i’m afraid they’ll break, but my gb4 feels so much more solid. i’m also a prtty big guy and a few pounds isn’t much of a concern to me. same with my trial uni though…
my trackmosnter koxx weighs a ton. but i find it is really hard to go unispins and to do hoptwists with it compared to my mates ultra lite kh24 wit ultra light components throughout. so i reckon it makes a difference when its loadssa light wegith stuff put togehter.
Not so much how heavy but where the weight is
Seems to me that the personality of a unicycle is most influenced by Wheel weight, crank arm length, and seat. Wheel weight (specifically rim, tube, and tire) is possibly the biggest influence because this is what gives you either a flywheel effect or a very responsive feel. The overall weight is not so much of a factor.
The rider is much more important than the unicycle, and weight isn’t the most important aspect of the unicycle, either.
But that being said, there are a lot of outstanding riders who never ride uphill, partly because their unicycles are so heavy. (Like the LA crowd). All else being equal, a lighter unicycle is nicer to ride.
For moves where you move with the unicycle (like hopping or hill climbing) it will have a minimal effect because of your own body weight. If you weigh 150lbs, upgrading from a 15lb uni to a 10lb uni (huge upgrade) will only yeild an increase of 3.125% (165/160…there are other factors which will probably make it more of an increase, such as the placebo effect and the ability to “tuck” the unicycle quicker). You still have to jump all 150lbs of you no matter how much the uni weighs, and thats what takes most of the energy. You will think you notice the weight, but will not really notice it that much.
Where you will notice it is in tricks where the unicycle or part of the unicycle moves seperately from your body (unispins, crankflips). But you can still do anything on a 15lb uni you can on a 10lb uni. You will get used to it quickly, and then if you come into some cash you will be that much better when you upgrade.
Five pounds of unicycle weight is not even close to equivalent to five pounds of body weight. The unicycle’s weight is at the end of your extremities, and it’s something you have to constantly adjust for. Try going jogging with a 5-pound weight strapped to one of your wrists or ankles; you will notice it. Strap the same weight to your belt, and you’ll notice it but not nearly as much.
A super-light unicycle doesn’t go anywhere if it’s broken. In other words, light is nice, but not at the cost of strength. In general, I prefer a unicycle that’s a little heavier if I can have it be very unlikely to break, especially for unicycles to be used in shows. The only place I really put effort into lightness was in my track racing unicycles. On my really nice, light one (stolen in China in 2000) I would avoid all hopping, curbs, big bumps, etc. This is fine, as it was intended as a track racer.
I also have one of the lightest MUnis out there, my Roger Davies Carbon (1998?) which weighs 11.2 pounds according to a digital bathroom scale. That’s including the suspension seatpost! It’s a beautiful machine, but I hardly ever ride it anymore because it only fits up to about a 2.3" tire, and the resulting wheel isn’t anything like as strong as what’s on my other MUnis. And it has a conventional square taper axle. It’s great for offroad racing though.
If your intended riding, which you didn’t identify, involves lots of hopping, jumping or big bumps, consider strength first, then weight later.
I like medium heavy unis because thay have a good balence of control and hopping height 
Also I find I can hop higher on my 24" than on my lighter 16" 
I used to play bastketball with a 5lb ankleweight on each ankle…
I understand that it is not the same…rotational weight especially. I also agree that a lighter uni is a nicer ride. But I would say when you put all the factors together 5lbs maybe gives you 10%. The example I was picturing and tried to describe was a sidehop, where you are moving from one point to another powered solely by leg strength overcoming gravity. Where you will find the most difference is XC riding, where the extra weight needing to be moved in a circle will tire you out quicker. He asked what tricks you can do and whether they are substantially harder.
With the ankleweights on I tired quicker from running, but I could still jump as high.
Obie, I love you and everything, but there’s no way that this can be true. Did you actually test this with one of those jump-height measurement boards?
To the OP - as JohnFoss said, you need to say what kind of riding you’re asking about for us to answer properly.
I ride mtn-climb road rides, and every extra pound adds to my finishing time. As Obie and others implied, however, your own weight can make a bigger difference than the few pounds’ difference in uni weight.
I agree that a heavier unicycle generally won’t change what it is possible for a given rider to do. When I swap back and forth between my 26x3.0" Yuni Profile setup, and my KH XC 24x2.6", which is probably two pounds less, the biggest difference is in how quickly I get fatigued on strenuous rides. The weight difference is the most noticable when I’m pushing or hiking the thing uphill.
Board? We used slow motion photography and a marked wall. I first had NASA come in and they measured out the increments. They used light to measure. The highest point was 1m, which it turns out is the distance light travels in 1/299,792,458 seconds. We found that I could actually jump almost 3 times as high with the ankle weights. The NASA people were very helpful. There will be a full article in The International Journal of Medical Sciences next month. 
Think about it. My feet are planted to Earth. The amount of force I generate downwards is the same, so the ankleweights do nothing until I leave the ground. When I do leave the ground they “abosrb” some of this force because the same amount of force is moving slightly more weight. But they have no affect on my ability to push on the ground.
220/230=.9565
So I can jump 24/25ths as high. I would say that the margin of error between my jumps is more than 1/25. So technically I jump a little bit less, but if I can jump 24 inches, its less than an inch.
Look all I know is I could barely dunk on a specific rim without them and I could barely dunk with them. Now I cannot dunk at all. 
Edit: Back on topic: Unirunner, buy whatever uni you can afford. You will enjoy it.
I know at the Moab MUni Fest in 2005 my Torker 24" weighted about 20 pounds with a brake. In 2006 my KH 24" was under 15 pounds with a brake. It was night and day easier to ride on those long hilly trails. I may be riding a 11-12 pound 26" this year. Right now my KH 29er XC is about 12 pounds with the brake. It is so much fun to ride on trails.
Weight is very important off-road.