What is more intuitive? Using the C or the Z?

In Neocasteyano 2001 and 2010 I thought it was better to use C for the voiced sound, as in zapato or cereza. However, in the 2010-JAP revision that I am making to make Neocasteyano compatible with Japanese rōmaji, it might be a better option to use Z, since it is the letter they use for the Z sound. I don't mind using C or Z, and this way I make Neocasteyano more compatible.