The other reason I use pt Sai
over artrage in some cases is the ability to easily trace a sketch with clean lineart. It is done by a lineart layer,in which the user does not have to even use a tablet (although it can be used and its handy for smaller details). That is done by drawing straight lines and making them curvy by clicking and creating manipulation dots. All that has to be done is point out all the correct places of the dots and the lines just snap and are later easy to manipulate.
This layer is by no means a Vector layer, but it does act like one and gives the user the power and ease of vector tracing. Lines do not act like in a vector programme.They are far easier to manipulate. The user can then tell a line to be thicked or thinner,by specifying its weight on a dot. Lines can also be colored,changed in size,they can look just like drawn by a skillfull hand. Lines are easy to erase and trim (alt+click on dots) without the danger of erasing other details and the need to change the options of a eraser tool.You just click alt+click on the dots.
So yeah,this is VERY usefull and saves a lot of time to anyone who is drawing- no matter in what style.
This function and the select/masking make paint tool sai so much better than many other apps. I would love to see it in artrage one day.