Based on the iPhone versions, I prefer Brushes. It has a stronger interface and better workflow in my opinion, and it's much faster and fluid to work with.
Sketchbook has the great line taper effect though, which makes it a good tool for line drawing, and you can make great sketches with brushes available. Easy to do shading. It also has a higher resolution canvas (although they are same on iPad). However, Brushes also has the (Mac only) Brushes Viewer, which allows you to export your art to 6X resolution. Great for prints of your work.
Basically I think they're both outstanding apps, and can be bought together for under $20, and that's what anyone interested should probably do. Although it is hard to really say since no one has used these apps or even used a stylus with the iPad yet.