Actually, it appears to me that the Atlas has better overall ventilation than the quantum and that the strip of metal on top that has no holes on the Atlas is pretty thin and likely won't trap hardly any heat. I didn't see a picture of the top of the quantum so i can't tell if it does or doesn't have holes on top. Also, if you consider the traditional plastic AR type handguards, they only have vent holes on the top and bottom (nothing at all on the sides) and must surely trap more heat than the Atlas and that doesn't seem to be much of an issue unless your really burning up your barrel. I think even with the Atlas being slim and closer to the barrel you wouldn't have any issues with it getting uncomfortably hot. The quantum is cheaper though and still looks good. I'd just go with which ever one you think you'll like the look and feel of best
And i guess difference in weight could be a consideration as well.