Generally I would go with .NET Core - it is still in its infant stage, but by the time you have MVP it will be OK - Microsoft are throwing a lot of efforts in this direction.
For the mobile apps - Xamarin. You will still write C#, just use a different way of thinking.
Having the same technology stack in web and mobile will allow you to reuse skills one day when you build a team.
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Generally I would go with .NET Core - it is still in its infant stage, but by the time you have MVP it will be OK - Microsoft are throwing a lot of efforts in this direction.
For the mobile apps - Xamarin. You will still write C#, just use a different way of thinking.
Having the same technology stack in web and mobile will allow you to reuse skills one day when you build a team.
Just FYI, .NET Core is at 1.0 and already has a LTS version. It is definitely production ready now.