what kind of graphic layout you want your
drawings to look like???
If it is a 2D image i think using only photoshop would be easier.
Ok the big difference with photoshop and painter are obviously
the brush pallate.
As you can see photoshop only have a few toolbar for coloring
for example

But if you use painter for example u want to use crayon.
Just by picking a crayon you could have so many types of crayon like
below

u have to try both to know which one suits you best.
Sometime you could have the same effect of a painter in a photoshop
and vice versa.
if you dont believe me you can go this link
http://forums.cgsociety.org/forumdisplay.php?s=a4fa2ad3cf18c4df4630a979f29ec584&f=133these guys are the best artist!! by clicking on which ever picture
it will show what kind of media they are using.
Mostly are using Photoshop and painter.