Battery choices...why?

We are getting our bearings on putting our 70 Safari together. It is pretty much stripped inside except for the bathroom. The wiring is supposedly new as well as the plumbing. The wiring looks like a rats nest to me. Anyway, we are going to have the wiring straightened out and we will need the batteries so that those can also be installed at that time. We have the urge to use solar panels installed on the roof in the near future. So, my question is what drives the decision to use one type / voltage battery over another? Why use 6v vs 12v? Why golf cart battery vs AGM or Lithium. I will need to make a decision soon on the batteries that we are going to be using and it will be one of the first big purchases for the TT. Thanks for your patience as we are very much newbies to putting together an elegant and functional AS. Tom