使用MongoDB和SQL Server Express与Asp.net MVC

I am building a website with Asp.Net MVC 4 and C#, hosted in VPS. The site has lots of sharing comments/replies features (I need to store a lot of comments). I was planning to use MongoDB as its free and also very suitable for storing blogs/comments, but got a little hesitant I am building a website with Asp.Net MVC 4 and