Lester

Having only 10 - 15 connections I would go for Superserver on Linux. I have
no particular reason, I have only the feeling that SS is more 'user
friendly'.

I have other sites with Firebird serving from 5 to 50 concurent connections
and this is Superserver working good. I would choose CS for higher amount of
connections and for dual- or quad- core machines running on Linux to get
benefits from multicore architecture (before Firebird 3.0 comes)

That's my 0.2$

Marcin