I’ve been thinking about Kedar’s post on the issue, and I am compelled somewhat bizarrely by this form of pedantry. My compulsive use of Google has unearthed the following:

The vulgar Latin that was prevalent in large parts of Europe in the centuries post-Rome, when the term was likely adopted, would have pronounced it more closely to the way it is pronounced today (ser-shee-rarr-ee, alternatively ser-shee-oh-rarr-ee). How originalist shall we be – as far back as the inventors of Latin, or only as far back as the inventors of the term?

Furthermore, as Merriam-Webster places the origin as Middle English, as opposed to Latin, and as U. Michigan acknowledges usage in Middle English, likely the soft-C is to be preferred. (By the 13th century, a c-vowel combination could be granted the soft C pronunciation. Source.)