You can write it in whatever makes you feel comfortable. However, I think you should consider; what software your supervisor/department likes, can you do the required formatting and will you exceed the 50MB docs limit. I would say most formatting requirements (journals, thesis) are written around MS Word and LaTeX. Google Docs is not that common so you may have issues with people accepting the file format. I would recommend LaTeX as you have complete control over the formatting and doesn't have any compatibility issues.