I think the obvious answer would be "neither" but for the sake of this writeup I'm going to eliminate that as an option. I have lived most of my life in areas that experience all seasons, none of which are generally all that extreme outside of rare and temporary parts of each season.
Starting about 2 years ago I started to take my health a lot more seriously and most of this was because I let myself drift too far down the path of being unhealthy. Most of what I changed was what I eat and drink but later a big part of it became that and also putting exercise into my life on an almost daily basis. Some days are easier than others and for me the hardest time of year to get motivated to get out and get my body moving is summertime.
During the summer it is here, like most places, extremely hot and humid and this is true most of the day, even in the nighttime as the residual humidity is still hanging around. In these runs for me I find it very difficult to keep moving because I become extremely uncomfortable and by the time I finish (which is normally a lot sooner than other times of year) I look as though I have been swimming rather than running. I look as though I am in terrible shape and when i quite after something absurdly short like 3K I sometimes feel as though I am not making any progress.
There are some websites out there that claim "your body will acclimate to the heat and humidity rather quickly" but I gotta call BS on that. I run nearly every day as I strive to get at least a mile in no matter what and I am NOT acclimating. Many days I feel as though it is getting worse. But that might just be me.
For me I really struggle in the summer months but then once fall comes and even winter, I excel and realize that my time spent doing what appeared to be almost no work at all due to my short runs in the summer were actually preparing me for fantastic performance once the weather is more to my liking.
Maybe it is just my biological makeup but once the weather turns those barely 3k runs turn into runs that are 10k plus without even getting terribly winded. Even in the summer I normally quit not because I am tired, not because my legs are giving out, but because I am so hot that I am struggling to breathe. There are many dangers associated with running in the heat but there are very few in the cool and cold.
If anything cold running encourages me to run harder because of the fact that it has the added benefit of keeping me warm. There are many people i know that run with a "cool band" or put a bandanna around their neck filled with ice cubes but i'll tell you this: In the summer heat those ice cubes are GONE in like 10 minutes.
Personally I still sweat in the cold as well but obviously significantly less and this to me is far more comfortable. If I had to choose to either have it be cold or hot every time that I go for a run it would be an easy decision: I would choose cold without hesitation. I can always put on more clothes. There are only so many you can take off without getting embarrassed or arrested.
Which do you prefer?