http://www.iosuserfeedback.com
I don't develop iphone apps but I immediately see value in the service. That being said...
I'm curious as to how you present the side-by-side view. Much of the content in the iTunes page is (directly) out of one's control (Customer Reviews, Customers Also Bought, Ratings etc). So you would be comparing primarily the logo and the description. It would be great if there was a way you could blacken out/shade the areas someone can't control so that what you can change (and therefore what is being tested) can stand out.
Good luck!
To your point about much of it being outside of your control... this is true; however the little bit you do control (that tiny description) can make a big impact. I see the service as a way to test the limited copy of your app page.
As a side note... I ran a test with one of my own older (a quotes app) and over 50% of the respondents said they wouldn't pick my app because it was simply randomized quotes instead of categories (competitor) so I did get some insight as to what my potential user values more from a feature perspective as well.