When I am writing on my smartphone, I use a word counter Notepad to write my contents. That is because I have to monitor that my content is well structured to fall in line with the required word count. There are times though that I just get inspired and start writing on the submission pane too, but the use of a word processor always makes it easier for me.
On the PC, i get to use Google Docs. It is intuitive and has good tools to enhance my writing tasks.