Personally, I'd recommend a 12V model over any 6V, especially when dealing w/ (wet) weeds or grass touching the popper wire. The model I went w/ was a Vanguard Red Snap'R (which can use either 6 or 12V dry/wet cell batteries). You can hook up a solar trickle charger to maintain top performance.
In my particular situation, I have a 6ft tall open top run which has a 7ft privacy fence as one of its walls. Because the coons were more likely to enter from the neighboring yard on top of the fence line, I wound up stringing wire tape along the top of the fence (where they gingerly balance & walk) and along the top of the run's wood frame. I grounded the chicken wire walls of run to the ground post(s), as well as wrapped a strand of heavy gauge copper (attached to the grounding post) around the adjoining tree limb (which provides vertical access to the middle of the run).
Without a ground connection, coons (or any other climber) can walk atop the "hot" wire w/out issue. Once they touch a grounded connection (chicken wire wall or copper wrapped limb) while perched on the hot wire, all hell breaks loose. Poor Mr. Coon launched about 2ft in the air when he completed the circuit from atop the fence while reaching for his climb down limb.