Which will be more supported or required by community?
For most business users, and most dedicated ASP hosting providers, it will be Microsoft SQL Server IMHO. With a surprising spread across version numbers. I see quite many SQL Server 2008, a lot of 2005, and quite surprisingly many 2000's too.
For small "we're cheap, is it gratis?" businesses and individuals, I do see some MySQL on Windows too. I guess in my little circle of companies that I've worked with, it is 2/3 MSSQL and 1/3 MySQL of the two databases you mention.
If we use MySQL, will it be a tough road ahead?
In terms of more difficult development? Yes and no; LINQ to SQL is more or less only available for MSSQL, and many find it to be a simple and natural technology to work with. So "yes" if your plan was to use LINQ to SQL as your ORM of choice. Other good ORM's like nHibernate work equally well with both databases, as elder_george says.
Is there any chance of supporting both by using ORM (I have no experience of working with any ORM)
Sure, f.x. with nHibernate, and a bit of extra XML for nHibernate configuration, and some more extensive install docs, SQL scripts et cetera for the users.
Any other suggestion ?
Since this is your first application, play around with the design first using mocksups & prototyping, UML or something else of your choice. See if you can get some experienced MVC developers to critique your design, but don't take their critique as gospel, there is more than one way of doing things right.
Definitely consider tvanfosson's suggestion to use an IRepository to provide flexibility on the storage layer.
Consider using unit testing and dependency injection from the start too; both are IMHO very important and helpful in the longer run. On the downside, that would be a lot of new technology if you don't have prior experience with these.