If you're planning an installation or an upgrade of your systems, make sure you think about the right edition while you're in the planning process. This includes planning for something that uses SQL Server, such as Sharepoint.
As you're aware, there are multiple editions of SQL Server, even since the earlier days. The choice of editions is largely based on three factors:
- How much work the server will do
- How many features you need
- How much money you have
If you are considering an edition for production, you need to make the decision carefully. Some people simply look at the cost of SQL Server, and opt for the lowest one. That's a mistake, and it will bite you in the end. Cost is certainly a factor, but if you run out of room in CPU counts, Memory, number of clustering nodes and so on, you'll end up not solving the problem you bought the software for in the first place. And even the most expensive SQL Server is WAY cheaper than many other vendors, so don't let that be a single bounding operator for you.
The various editions have different feature sets as well. For instance, Compressed backups are only possible in Enterprise Edition, although you can restore a compressed backup from any edition. The point is that you need to be familiar with the various feature sets and where you find them.
You also need to think about how many users and transactions the server has to handle. The higher the edition, the more work it can do.
You can find out which features are in each edition here: http://www.microsoft.com/sqlserver/2008/en/us/editions.aspx