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A main difference between the two is OpenStack is an open source Rackspace spin off adopted by major vendors such as H-P, Citrix Systms and Dell. Eucalyptus just does not have the same level of top-tier support. Heck even NASA has used Rackspace. But OpenStack is really still in a development mode, and I don't see it as quite at the point where it is inevitably going to become the standard open source cloud platform. That is not to say it will not, just that the tea leaves still have not revealed the future.
Eucalyptus has one thing going for it that OpenSpace does not, and it is a big thing; Eucalyptus is designed to be an API that is Amazon EC2 compatible. Since Amazon is a HUGE player in the cloud that alone can be enough to make the decision between the two products for some companies. Incidentally, NASA used Eucalyptus as well, until problems with scalability became and issue for NASA. Eucalyptus also differs from OpenSource in that it is not truly open source, for good or ill. Eucalyptus has features that OpenSource does not yet offer, such as VMware compatibility and SAN integration.