Open Canvas is very good for line art and shading. it has better blending and more accurate tools than Photoshop (in my personal opinion). I use Photoshop for flat color, lighting affects, and image clean up. Open Canvas has more options in tools which gives better lines, and really good blending for shading. SAI is much better than Open Canvas but you have to pay for it. If you can I would suggest that it is worth the money. If you have a tablet you can also check your pressure sensitivity.
hope that helps some :]