Ok, I was going to use 12 volt and decided that wasn't such a good idea, then 6 volt (but with quite a bit of extra weight). Now I am considering 8 volt as kind of a compromise. Any thoughts on this? I remember someone saying that the 8 volt batteries were weaker somehow.
OK lets bust your first misconception. The key is energy density expressed as wh/kg. What I am driving at is it doesn't make any real significant difference if you use the 6 volt or 8 volt.
The T105 is a 6 volt @ 225 AH or 1350 watt hours.
The T875 is a 8 volt battery @ 170 AH or 1360 watt hours.
Both batteries are the exact same size.
L= 10-3/8 W = 7-1/8 H = 10-7/8
T105 Weight = 62 pounds.
T875 Weight = 64 pounds.
T105 Energy Density = 48.21 wh / kg
T875 Energy Density = 46.89 wh / kg
So in this case the T105 has only a slight advantage.
However there is one difference of note to be made here. The T105 has a higher AH capacity (225 AH), thus a lower internal resistance. What this means is the T105 can deliver higher amounts of current with less voltage drop or SAG. So IMHO you gave up something going with the 8 volt battery.