The SGP and power support are equal. The monoprice has a slight rainbow effect when the screen is OFF, but when the screen in ON it looks exactly like an SGP/PS.
I beg to offer a different opinion with regards to SGP Steinheil Ultra Crystal and the PowerSupport Crystal Clear.
Under normal condition, they look the same and feel the pretty much the same. However, thing is different when you look at a light source.
When looking at a light source from the screen, the SGP has more prism effect and you can see some blur/multiple images. (there is no rainbow effect on screen though)
I took it and let my coworker sees it and he also commented there is some slight wavy look from the SGP one.
I have the Power Support CS on iPhone4S and the SGP Steinheil UC on the new iPad.
Also for installation, I find the SGP one has more static and it is more difficult to install when compared to PS, especially when you have to re-align the screen during the installation. Also the adhesive is not as good as the Power Support (but SGP has improved, compared to a previous SGP clear one that I installed on 3GS)
The particular SGP one that I received also has a very tiny adhesive defect in the viewing area (ie. inside the black/white frame) where you will mistake it as a bright/dead pixel. So I have ordered a Power Support CS..
On another note, the SGP Invisible Shield (Ultra Matte) for the body of the new iPad is quite good in terms of texture, ease of installation, and alignment/cut design/coverage. It is similar to the matte finish from Wrapsol.
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It's called MARKETING. Also there is fancy packaging, sales offices, salesmen commissions etc etc. Then there is also plain old BS hype. They know that the majority of "iPeople" will pay through their ears.
The quality of Power Support or SGP are definitely better than the one that came with my SwitchEasy case. So I am inclined to think the PS or SGP are worth more. But the PS pricing (US$30) is on the high side.
Again, it comes down to expectation as some people don't care about rainbow on the screen when the screen is off.

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Let's throw in Martin Fields aswell!
Martin Fields is pretty good as well, in terms of clarity. I had it on Nokia N95, N800, etc, many years ago. Two problems I encountered a few years ago were ---- the adhesive is simply too strong for you to remove the film. It took many scotch tapes to do the job. Secondly, the price was simply too expensive. (it is even more expensive than Power Support)
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Well I say trust monoprice, I buy tons of stuff from them. HDMI cables, for like $3, they have everything cheap, they really are great. I say go for it, you can prob get 25 of the monorpice ones, compared to one of the others. I love the ones I have had on my phone, if I wasn't so anal they would have all lasted a good year on the phone and looked good.
Monoprice is a good company to deal with from my past and recent experience. They always ship things quickly on a consistent basis (talking about shipping to Canada) and the C/P ratio of their products is high. I have bought a lot of HDMI and Cat5E/6, Apple sync cables from them but for screen protector, I am looking for the best one on the market since I am sensitive to quality of film on a screen.