Photoshop is an image editor - it's main raison d'etre is for editing photographic images, and for creating raster graphics for web pages, or for print. It's professional image editing software - the industry leader in fact. Although it can be used for drawing and painting very successfully, it is not a dedicated drawing program. In fact, it's total overkill for that purpose (not to mention expensive). Also it's missing some of the functionality that dedicated drawing software has. Therefore, it's definitely not the best software out there for that purpose.
Paint Tool SAI is a raster drawing/painting application - specifically designed for that purpose. It's of little use for editing photographs. It's focus is on digital art only.
Paint Tool SAI does have some issues though - for a start it's not free, and the application is old and hasn't been updated since 2008 - and you have to rely on a fan-made hacked version to get it to run on modern operating systems, and it only works on Windows computers. Even though, it's still very good at what it was designed for.
Krita is an open source drawing/painting application which is still being actively updated, plus it works on Windows, and Linux computers. And even better, it's free http://www.krita.org