At $4.99, Sketchbook Pro is mind-bogglingly powerful
Sketchbook Pro, for the princely sum of $4.99.
For my needs, it was definitely worth the upgrade. The ability to export a layered file is critical for me. But for a lot of people, I suspect that Sketchbook Express is all you will ever really want. And I continue to be impressed by what a full-featured app the free Express version is - it has probably 80% of the functionality of Pro, including the ability to scale, move, and rotate a layer.
Sketchbook Pro offers a ton of brushes, compared to the default 15 that come with Sketchbook Express. But as is so often the case, 95% of the new brushes are useless gimmick brushes, like leaves and sparkles and stuff. There are a few paintbrush textures that I have enjoyed using, but for the most part, I think the default brushes with Express will be enough for any casual user.
These new brushes are difficult to select, as well. You have to swipe your way through a bunch of panels to see them all. Or you can tap a teeny tiny dot to go directly to the page, which I can never manage to hit.
Sketchbook Pro offers much better export capability. Whereas Sketchbook Express only lets you export to your photo library or print, the Pro version lets you export to your photo library, iTunes, Flickr, Facebook, Dropbox, Twitter, Email, and Print.
It also - and this was the "killer app" for me - lets you export it as a layered PSD file. This way I can sketch on my iPad, export it to Dropbox, pick it up on my desktop computer, and finish the fine tuning in Photoshop.
Sketchbook Express lets you have three layers. Pro lets you have more layers… but only up to six. Why only six? I do not know. But I suspect that most users would find that three layers is more than enough, given that you have full functionality with them.
(For new users unfamiliar with layers: sketch out a basic outline on one layer, adjust the opacity about 30%, thus creating your own tracing paper. Now add a new layer, and start drawing on that, using your first layer as reference. You're welcome!)