Pleasure, the decision to go SQL Server Express takes a few simple answers:
1) Free SQL Server 2005 vs Free SQL Server Express 2008 ... 2005 is well founded, 2008 is new and if the data is critical, I like to go with the solid well used...2005.
2) The SQL Server Express 2005 bundle comes with Management Studio. With 2008, you have to use the 2005 Management Studio. (2005 has more 3rd party tools as well...not to say they won't work with 2008)
SQL vs JET
1) Sql Server has far better security for your data!
2) Sql Server is more complicated to use if you need to develop code behind the forms. Basically you have to learn more things. Access/Sql Server/ADO/T-SQl/VB/DAO. With Access you need to know Access/VB/DAO at most, and for simple non complicated applications, just Access.
3) Data volume. Access is limited, SQL Server in most respects is not. Access is limited to 2GIG, which is a heck of a lot of data, so this may not apply.
4) Performance....Jet is faster with smaller databases...SQL Server is faster with larger tables when doing searches. Both are equally fast on single item lookups and updates.
5) Field level security....with SQL Server views, it is easier to permit restricted access to user groups, right down to the field level.
6) Access does not pull all the data back from SQL Server to do functions. This only occurs when the code (written in vba) cannot be translated by Access into a valid SQL Server request. Even then, the SQL Select/Insert/Update can be delivered to SQL Server via a Pass Through Query, which bypasses the Access translation process and delivers the SQL request directly to SQL Server. In addition, SQL Server can have stored procedures which can be executed and run on the server with only the result set returned.
Long and short...Small to medium databases with average to complex queries and intermediate or less programming skill...use Access. Good knowledge of VBA/DAO/ADO/SQL Server/T-SQL or the ability to use the forums well...with large databases and some very complex queries...SQL Server.
Smiles
Bob