Dual 6 volt battery's will give you a little more AMP hours and 6 volt are said to last a little longer. But since you have a good 12 volt already I would go with another to save some costs. Another thing I like about the 12 volt battery is I can use it in the truck if needed. I have other ways to charge a battery but you just never know when a battery is going to give you problems especially going from 70 degree temps to below zero. Not the time I want to figure out my battery was weak and not have a backup. I also take a couple batteries out of my boats and gang them together when I am winter camping for any length of time so 12 volt works better for me.
