I recently saw 'sibilancy' in an article wondered what was wrong with 'sibilance'. It too is apparently accepted though.
Do we really need so much choice? Any votes for a favourite here?
And finally, are there a lot of cases of such closely related synonyms?

" I recently saw 'sibilancy' in an article wondered what was wrong with 'sibilance'."
"Sibilancy" to me suggests an abstraction. "Sibilance" is the real thing. "Sibilation" might refer to the process of producing the "s" sound.

Merriam-Webster OnLine does indeed list sibilate in transitive and intransitive senses meaning to utter a sibilant sound. It also lists sibilance, but no sibilancy.

I'll not mention Ænigma's apparent fixation on a certain Sibley

I too had unterstood sibilation as referring to the 'process of producing the sound', as you put it Bill.
But I'm not sure I understand your idea of abstraction.. do you mean when hissing is understood in human terms to be jeering or generally showing one's dislike? Would you care to give an example?

Dear belligerent yourh: For many years I have been afflicted with tinnitus aurium. At first it was like tree frogs in spring heard all year around, Then some cicadas joined the chorus. Then all music began to sound out of tune. Now in addition there is a constant sibilance like steam escaping from a radiator. I have trouble understanding speech because the consonants require frequencies I can no longer hear.I have a two thousand dollar pair of hearing aids, but they help so little I do not use them.There is no sibilance anyone else can hear of course, but an
illusion of sibilance, a sibilancy produced by nerve damage.