I'm developing website for online test/assessments and I want to know what database is best for my project between Mysql and MSsql. There will be a lot of users writing concurently/ similtaneously to database.
Both products can be good at what you want to do. MS-SQL costs more up front (the purchase price). MySQL costs more day-by-day (personnel to keep it running acceptably), although if you plan to dedicate people to supporting MySQL anyway, then that cost can be negligable.
At least in my opinion, find the database engine you are comfortable with and use it. I haven't heard anything in your description that makes me think any one engine has a significant advantage over the others.