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I haven't seen research but if you've got the skills to pass all the exams, go for the MCA (Microsoft Certified Architect) for Exchange 2010, SharePoint Server 2010, SQL Server 2008, and Windows 2008 R2: Directory. This requires obtaining certification first as a MCM (Microsoft Certified Master) in those areas. Obviously there will be more demand for people who know SQL and Exchange because of the number of businesses that rely on SQL and Exchange, but because of the rarity of Sharepoint Experts, that could prove to be a more lucrative career path.