I came across some information regarding BizTalk 2004 and WebMethods. You can view the document at http://cs.jaxdug.com/files/14/reference/entry1237.aspx.
The document is a high level overview comparing BizTalk 2004 and the WebMethods suite. I’m not positive which version of the WebMethods suite it compares but it appears to be the current version. Overall, the document does a good job (at a high level) explaining the differences as well as explaining the strengths and weakness between the two systems.
Interestingly, scalability is not mentioned in the comparison document. Scalability is most likely the second question people ask when comparing BizTalk to other tools. The first question is usually “What’s BizTalk?” or "Isn't BizTalk a message standard like ebXML or EDI?"
It is probably best that the document does not discuss scalability. After all scalability is a vague term and it not easy to measure. A better term is Price/Performance (TCO divided by the Transaction Throughput). The trick, of course, is how detailed do you get when calculating TCO and how do you define a transaction? It would be nice if we had something like the TPC-C benchmark but for Business Processing/Message Integration. I already have a name for the benchmark – TPC-M (hey, you heard it here first). However, since a TPC-M benchmark does not exist you will need to do your own due diligence when comparing the two systems. Keep in mind that Microsoft products usually excel at providing high value and performance at a low cost (i.e. IIS, SQL Server, Windows Server, .Net, MS Office, etc.). BizTalk is no exception and is built on top of IIS, SQL Server, Windows Server and .Net.
To assist with your research here is a link to an HP document (http://h71028.www7.hp.com/enterprise/downloads/MicrosoftBizTalkServerRecommendedconfigs.pdf). The document demonstrates the scalability of BizTalk 2004 from a simple architecture (1 BTS server, 1 SQL server) processing 190,000 2k documents/hour to a high-end high-availability SAN architecture processing 1.5M 2k documents/hour.
In my opinion, compared to what I’ve seen from the other players in this market IBM Websphere, BEA WebLogic and Tibco. It is hard to beat BizTalk Server when you take into consideration its functionality and price/performance.
Note: BizTalk 2006 (currently in beta) is a much improved system with support for 64-bit processing and several other enhancements. I’m sure the Price/Performance will be even better.