Others have mentioned transaction capabilities in MySQL as a reason to choose it. I agree.
I also suggest you evaluate comparative deployment costs for web properties based on MongoDB vs. MySQL. There are dozens of hosting services offering MySQL services; competition is fierce between them; MySQL has years of experience delivering decent quality multi-tenant service; MySQL is widely understood among tech support people and network engineers. You say this is a personal project. Unless one of your purposes is to learn to configure and administer your own web property on your own, you should consider using the more widely deployed technology.