SME and SQL Express
SQL Express is limited in some important ways:
1) The amount of memory that can be used per instance is limited to 1 gig. (Note that because we do processes outside of SQL in the operating systems Common Language Runtime (CLR), the actual usage will be above the 1 gig limit. I’ve seen Express take up to 1.5 – 2.0 gigs of memory so MS is not consistent in how they throttle it).
2) The CPUs are limited to one. (In 2012 it is “The lesser of one socket or four cores”. (this is an improvement over earlier versions but still a limit)
3) The database size is limited to 10 gigs.
4) Express versions also go into ‘idle mode’ if the database is not used for a period of time (means the first person to start using SME after a period of inactivity, companywide, will notice a slower program until SQL rewrites the cashed query processes which improve speed).
Given those restraints we would advocate going with a full version, if you can possibly afford it. The licensing for SQL is a bit of a maze to work through so you may need to work with someone that knows the best license scenario to purchase. Standard Edition should be adequate.