It's worth remembering that Photoshop was not, specifically, designed with illustration in mind. They've done a lot in more recent releases to make it a friendlier drawing tool, but it was originally formulated for photo manipulation and graphic design.
That being said, the best art program for you should be determined by your interests.
If you want to make comics, get a copy of Manga Studio EX.
If you want to make painterly illustrations, get Corel Painter, ArtRage or Paint Tool SAI.
if you want an all-around workhorse with basic capability in multiple tasks, get Photoshop or GIMP.
If you want to do 3D modeling, get Mudbox or ZBrush.