Easy layout to follow, wide variety of customizable brushes, light program that won't take much space or memory to function properly. What I love most about Paint Tool SAI is the pen pressure, unlike Photoshop, Sai uses the complete range of pen pressure from the tablet and offers an adjustable stabilizer tool which allows to create smoother lines.

The sensibility that you could give to the pincel and the liberty in dragging the canvas

There are two features in Sai I prefer the most: 1. Brushes. I don't know why, but I really can't make coloring as soft and smooth in Photoshop as I do in Sai. Their brush settings really simple and obvious and to change them is really handy and quickly. 2. Stabilizer. This function helps me A LOT when I have to create very accurate and neat lineart. How I understand it, it is a speed of how your tablet pen draws the line. And again, it's so handy and you can go through speeds (0 - 22) easily. 3. I adore Flip Horisontally in SAI 4. I do like SAI has the variety of how to view color palette: color wheel, RGB slider, scratchpad, user palette and other. You can switch on two a more at once.