There is a new free drawing app for the iPad called Sketches. I downloaded it and it's great!
It has brush pen, felt tip marker and paintbrush as discrete tools. There's only one brush size for each unless you buy the Pro version (£1.49!), but the brush pen and technical pen make up for each other. It's quite sensitive, and you need to pick the option to undo via button because the 'backwards swipe' undo just makes a mark on your drawing. But I like it much more than Paper!