Tough recommendation, If you want your artwork to look professional, you can't use Sai. I haven't seen any professionals use it outside linework, and even that is rare. I do recommend it if you are a beginner and learning how to draw lines correctly/picking colors, etc. but eventually you will have to move on to a different program. Just in case you need proofs, ill show comparisons of my Sai artwork and my Painter artwork. I don't mean to write this to criticize your recommendation, I'm just pointing out it is a beginner's program. sai:. painter:
Which is an okay reason. My artwork before changing to painter was actually pretty good, but was bad for a while after switching to painter. Eventually it paid off, as you can see. You shouldn't really worry about changing until you're in high school, if you are going to go after a digital art career.