The Enterprise edition is the only edition that allows you to add >1 BizTalk computer to your "Group". If you need to "scale" for reliability and high availability, get the Enterprise edition.
So, what does this mean?
On BizTalk Server 1, there is a host with a host instance getting messages for processing. On BizTalk Server 2, you can create another host instance of the exact same host to get messages for processing. Now, there are two host instances getting the messages from the same location. If one of the host instance stops, you have failover with the other host instance. This functionality is built-in the Enterprise edition with no additional software or hardware requirements, including clustering. This functionality is not available with any other BizTalk edition. The BizTalk Evaluation edition is a time-bombed Enterprise edition.
If you require reliability or availability, then multiple BizTalk computers in the Group with Enterprise edition is the best choice. The Standard edition uses only one BizTalk computer in the computer. On the Standard edition, a host has one host instance on one BizTalk computer.
The following table lists some of the differences between the Enterprise and Standard editions:
>1 BizTalk computer
1 BizTalk computer
>1 MsgBox databases
1 MsgBox database
Limit of 5
Includes the BizTalk Adapter Pack (BAP), Accelerators and Host Integration Server (HIS).
All the BizTalk computers in the Group use the same BizTalk databases hosted on SQL Server. The differences between the SQL Server Enterprise and Standard editions are completely separate from the BizTalk editions. The BizTalk and SQL Server editions can be mixed. For example, you can have a BizTalk Standard edition computer using the SQL Server Enterprise edition. You can have a BizTalk Enterprise edition computer using the SQL Server Standard edition.
Pricing and Licensing FAQhttp://www.microsoft.com/biztalk/en/us/pricing-licensing-faq.aspx
Pricing and Licensinghttp://www.microsoft.com/biztalk/en/us/pricing-licensing.aspx