The best source of information on the planet about light aircraft electrical systems is Bob Nuckolls' page at www.aeroelectric.com.  His book is great.  The Aeroelectic List and its archives contain the best free technical support anywhere.  Read the posts carefully, especially those posted by Nuckolls himself, and especially the logic he provides for why his recommendations are as they are.  You won't be able to poke holes in his logic.

Based largely on information gleaned from this source, I chose a 12/14 volt system for my ES.  I don't have air conditioning, and I don't have so much wiring that the weight savings from smaller wire would be significant.  One benefit of going this route is the wide availability of components like overhead and external lights from the automotive industry.

Visit the site, think about the kind of electrical system you want (number of batteries and alternators, type of redundancy), use his templates to do a load analysis, and choose.