A few thoughts -

To make the same power, at a lower voltage, you need more amps. V*A=W. So, at the same power output, the higher voltage system will run cooler, and have less stress on electrical components.

Also - our batteries are generally rated in mili-amp-hours(mah). To look at total stored power, it can be useful to use mili-watt-hours(mwa) or watt-hours (wa). So if the cells are 5000mah, each cell has (nominal) 18.5wh, so a 4s battery would have 74wh, and a 6s would have 111wh. So if the watt load (power load) is the same for both packs, the 6s will last longer.

Personally, I have antidotal proved this out to my self - at my local track, I often run my 4s 1/8 buggy and truggy at 70% throttle. Once, I tried to run 3s at 100%. Performance on the track was y similar. Runtime was noticeably shorter ( I didn't time it, but was surprised when it LVC'ed). Also, my motor truggy motor normally comes off the track about 110-130, that day it was 160f - lower voltage needs more amps to make the same power, which means more heat.

My .02