isolatedstorage probably exists because the namespace is called
System.IO.IsolatedStorage. The concept is referred to in MSDN, however, as "isolated storage" (not even a proper noun!), which of course aligns with the tag, isolated-storage.
When presented with choices like these, I usually favor the name or title that is used in writing over the code identifier, so I'd go with the hyphenated tag. Either way, when the tags are synonymized, only one tag will actually be shared by and displayed in questions, whichever is entered in the Tags field.
Also since Bill is asking that a few people weigh in, we'll hold off on doing anything for the moment.