Having just reviewed the updated Snapseed app, I was anxious to see Photoristic HD, a newly updated app (US $4.99) that offers similar, but enhanced features.
Unlike Snapseed, Photoristic HD is an iPad-only app. It offers a wide variety of preset image "looks" and offers more precise control of several image parameters. It works in a similar way to Snapseed; you hold a finger down on the screen, and scroll up or down to see the image parameters, like saturation, clarity, vibrance and so on. Swiping left or right increases or decreases the effect you've selected. If you'd rather use the presets, you can see dozens of ways to present your image. With Snapseed, you can get the same result, but without presets and a lot more adjusting.
I was particularly impressed with the black-and-white presets. As pro photographers know, taking a black-and-white photo through different colored filters results in very striking and different results. A red filter, for example, gives very high contrast with deep blacks. Blue or green filters render the photo differently, and can emphasize foliage or sky colors. Photoristic HD has a wide selection of B&W filters for one-click application, while Snapseed has you moving sliders back and forth. I also liked the clarity feature, which can enhance landscapes by sharpening the structure of rocks, plants and clouds.