It is the purpose of a service pack to fix issue found in the official release. Thus, for service packs we primarily fix bugs, while major releases also contain new features and controls. In all cases we try to keep the suite stable and fix as many problems as possible, but it is simply impossible to anticipate and test the myriad of possible scenarios when creating a new feature. In this sense, perhaps you can consider service pack releases more stable simply because they contain a number of bug fixes that the previous release did not. This is also valid for any release, however, a major release would be considered more stable than the previous major release.
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