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(1) Buy both domain names.
(2) Use keyword1keyword2keyword3 unless there are some double letters or it becomes unreadable or confusing (say the end of keyword1 and the beginning of keyword2 happen to form a different word)
(3) Use the dashes if some confusion arises.
If the name of your site is Experts Exchange.
Would you want:
One is clear, one sounds...adult
Another example, I'm just making this up, so sorry if it exists, but say it was about dolls and their looks:
One is clear, the other has too many of the same letters squished together and could confuse people.
Unless you run into the issues above, you could go either way, although hypenless would probably be easiest to remember (as was said, register both, have one re-direct to the other).
That said, as others have mentioned: get both versions and redirect one to the other!