DotNetNuke purposely gives developers database choices so they can tailor the Framework to specific development needs or production environment requirements.

The SQL Express option might be used for mainly Framework development and web site Hosting. Most Hosting Companies I've checked out require SQL Express for their most basic plans.and many of them offer it as a database option everywhere but the highest demand Enterprise offerings.

SQL Express simply has a lower disk, memory and network cost than SQL Server and in working with Azure/Amazon clouds I think it may be around for a very long time to come.

SQL Server you'll find in Enterprise level applications where you need multiple SQL Server boxes for performance, security and fail over redundancy. 

As far as creating in one database type and moving to another you can do that now the SQL Express database can be re-created in a SQL Server environment just by backing up in SQL Express and then restoring into SQL Server and vice versa.
However, whether you can go backwards is totally dependent upon the type of database you have and the customizations done in it..

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Ken Grierson
DotNetNuke Corporation
Test Lead

I think no advantage/disadvantage for SQL express, the only difference is SQL express on local and full version sql server on production server (hosting company)

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