In the BizTalk Hotrod quarterly ezine. You can find a condensed version of the article at http://www.biztalkhotrod.com
The full version (huge!) can be found at http://biztalkhotrod.com/Documents/BizTalk%20Smack%20Down%20Full.pdf
The article was a ton of work but was well worth the effort. The intent was to have some fun comparing BizTalk and WF/WCF. Let me know if you have any feedback!