I don't use either right now, but back when I did use it there were a lot of things about Sai that just wouldn't work (Text being a huge one) for what I needed for or couldn't handle the file I wanted it to work with and crash (it wasn't a computer issue, it was the program not having the capabilities or memory or whatever)
I pretty much only used it for quick sketches.
I did not like doing line art in photoshop at all though, all my lineart would come out really shaky (mine was an older version, newer ones may not have this issue) I did pretty much all my lineart in Mangastudio4, and sometimes sai.
Now though instead of switching between 3 programs I just use Clip Studio Paint. It's options regarding text are less than desirable compared to Photoshop, but it works very well for what I need it, and it handles lineart and large files very well.