In this case I think all are good, so it would really depend on the other word...
For example, I personally would take DirtySecrets over DirtySecret because the first gives you more development opportunity, you could go on and on about all the dirty secrets...
????: NamePros.com http://www.namepros.com/domain-newbies/116169-secret-or-emotions.html
But, if there was one big huge dirty secret then DirtySecret would be cool, but the life span of a site might not be as long...
Also, if it was <Someones>Secret then that might be better, depending on who that someone was...
And the same goes for emotions, we all have emotions, not just emotion...
Anyway, I think they are both great words and both have development potential, it does depend what the other word is though...