If you're good with SAI then keep using it. 
Paint tool SAI is a good painting program for beginner artist.
It's simple and easy to use on desktop PC with normal pen graphic tablet like Wacom Bamboo or Intuos.
But SAI is old, and extremely buggy on any pen graphic display tablet. SAI 2 is still in alpha state.
SAI can't be used on high DPI display, can't create any giant size canvas over than 10000px, no touch support, no text tool, no file backup,... and more. It's nearly imposible to use on Wacom Cintiq or Surface Pro, or any pen graphic display tablet devices.
When you need it to be used as a professional artist, most of the time you can't use SAI alone, but must use an additional program to complete the artwork. Meanwhile, Clip Studio Pro/Ex has nearly all the tool you need for painting and creating comic.
I'm not trying to force you to stop using SAI. If you like it, keep using it.
I just mean to says, you can save more time if you can find a better tool to practice.