Gartner recently released the "2013 Business Intelligence and Analytics Platforms Magic Quadrant" as shown above. I suggest using Gartner's Magic Quadrant as one source of guidance - albeit with a heavy grain of salt. There may be conflicts of interest with Gartner's ratings. It is wise to give more weight to organization requirements and best platform fit than to vendor ratings. In addition, there is no perfect BI / Analytical Platform - each have strengths and weaknesses.
It is advisable to include data scientists in the selection process. They will provide invaluable information about the strengths and weaknesses of various BI / Analytical Platforms. Moreover, they will use different analytical tools for their job and know best how those tools integrate with different BI / Analytical Platforms.
Many organizations are electing to architect and build their own BI / Analytical Platforms using a mixture of open source and proprietary technologies. This has the advantage of greater technology agility and flexibility with the potential for lower costs. The disadvantage is greater expertise and time required to design, build and maintain the platform. Many value IT agility and flexibility for the future over other considerations. Others assign greater weight to perceived security of the black box of a major vendor.
I strongly suggest using a third-party professional - usually an independent professional IT services firm or systems integrator - to provide counsel in the selection process. They can provide an independent opinion - free of conflicts of interest - on which vendor(s) best meet needs of the organization. They can also help design and implement an overall Information Management Plan.
It is critical to have an organizational Information Management Plan to get optimal use of the BI / Analytical Platform. This may entail re-engineering knowledge / business processes and painful change management. Yet the short term pain may be worth the long term reward.