I downloaded Inkpad (an iPad app) from the App Store. It seems to work pretty well. The concept of a touch interface for vector drawing seems a little bit revolutionary compared to drawing with a mouse, at least for the freehand capabilities. It's also made by the same folks who made Brushes, which was one of the first painting apps for iPad. I found one list online and it was at the very top.
I chose Adobe Fireworks CS6 for Mac, because it is much cheaper than Photoshop and Illustrator, and it seems to do everything I need it to, and in the same way that Illustrator and Photoshop do it, which is a plus.