SMBs should consider using SQL Server Express as much as possible because it's free and has many of the same features as the other editions of SQL Server. However, as your client's trusted partner, you should be aware of the limitations of SQL Server Express.
If the needs of an SMB database exceed the limitations of SQL Server Express, the client should consider some of the other editions of SQL Server. The official Microsoft site linked above provides a list of limitations and features of the other editions of SQL Server.
Clients with a large number of users must purchase a per processor license for SQL Server, which can be more expensive than the hardware they are using. In general, I recommend that SMBs use database servers that will allow them to grow into and will provide optimal service for the lifetime of the applications using SQL Server. I normally recommend the 64-bit Enterprise Edition of SQL Server because its feature set scales to meet growing needs.
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