Many charger manufacturers have taken note of the increased demand on a battery to charge in cold weather conditions but high temps are just as important to note. While the voltage variants may differ from one battery manufacture to another a general rule of +/- 0.03 volts per degree C above or below 25⁰C per cell.

This feature requires a temperature sensor to be located near the batteries and or on the batteries. Which is best? Well if it's cold conditions you face often then between a pack of 2 or more batteries is ideal because the batteries will be cold in the middle. You could just as easily argue that if the cabinet is cold and the batteries were being used that they would still be warm internally which we'd have to agree with also.
You could also state that in high-temperature conditions the compartment would be hotter than the batteries so reducing the battery voltage prior to thermal runaway would be advantageous. Just as if you are monitoring the temperature of the batteries directly if there was a dead cell the battery would be hot and potentially gassing (which is bad). So early indication of this is best. But we'd add that a warm gassing battery would also heat up a battery compartment so the ambient temp would increase anyway.

In general, the thing this is really important is the upper and lower temperatures. Below Zero you shouldn't be charging and above 40⁰C you shouldn't be charging what happens in the middle is your applications use are an ongoing variable that either extends or reduces battery longevity.

It's important to note though that Temperature Compensation is the ability to automatically alter the charging voltage due to the environmental temperature the batteries are stored/housed in and not anything to do with charger temps, this is typically understood when an external temperature sensor is available and or provided.

Well I guess there's a few ways to look at this and considerations.

The largest 12 volt batteries are 260Ah (approx) @ 20Hr rate. 6 volt batteries can be 420Ah as they have half the plates but twice the capacity. So one answer might be yes 6 volt is better if you need 400Ah of battery because you could use two large capacity 6 volts instead of two mid-sized 12 volts. The 6 volt battery at 400Ah is quite often an L16 case size which is tall, these batteries in a flooded battery require the height to store them and also need to be deep cycled regularly to keep the acid from becoming stagnant (see stratification).

From an electrical point of view a 12 volt battery is actually six 2 volt cells in series. There is no difference between 2 x 3 cells (2 x 6 volt) or 1 x 6 cells (12 volt). The physicall size of these cells is quite often an issue for different installations so sometimes one form factor is better than another.

Lastly, if one of your batteries fail in a 2 x 6 volt installation your voltage drops dramatically (normally 2 volts per dead cell) or if there is an open circuit (just like a fuse blowing) the voltage will be zero volts. That may render your vehicle useless. Whereas if you have two 12 volt batteries in parallel the voltage will appear normal but your capacity will be reduced by 50%. This later is harder to detect if you don't have a battery monitoring system because you can't see the capacity reduction vs. voltage.