I have some recommendations.

First, I use ProCreate on the iPad every day, often for hours at a time. It's a fantastic raster-based drawing/painting application. I've tried virtually all of the drawing and painting apps in the App Store, and it's by far the best. You can easily use it for just "line work"...check out the quality of the line work I do with it: ChristianStiehl.com You can also export your images as JPEG, PNG, and layered PSD with transparency and everything. It has great DropBox integration, and active developers who are working on new features all the time.

If you want to do vector-based art, there are a few different options. Again, I've tried a bunch of the apps available. The one I kept coming back to, due to number of features and ease of use, is InkPad. You can save your artwork out as SVG vector files that you can open right up into Adobe Illustrator. You can even bring custom fonts into InkPad via DropBox so that you can do text on the iPad using typefaces that don't ship with it.