I've never used Xara, but as long as the underlying drawing or sketch is sound I would think either program would do fine.
My usual approach is to scan and then import in my pencil concept drawing, put it on it's own layer and lock it. Illustrator has a feature called 'template' that does this for you. Then using the bezier (or pen) tool you build the illustration up on the layers above by tracing your sketch below. Maybe put the background on one layer..linework on it's layer.....solid color on another....blends and shading on yet another. The idea being that by locking elements of the drawing onto their own layers you can organize your art so you are not fighting with too many vector objects at once. Giving each layer a unique color so you can tell them apart in wireframe view helps a lot too. (Illustrator does this for you automatically.)
