I don't think there is any question that it's all relative. But quitting because it is 30 Farh. is out of the question for those from the North.
Comparing running in bitter cold (which I would rate as anything under 0 deg F.) and in extreme heat (which I would rate at above 95 deg F.) is kind of meaningless. I believe it takes more toughness to run your "normal" routine in extreme heat. However, I am from Michigan. I feel it is harder because Winter is colder no matter how cold it is right now so you are generally more used to it. Also, with the right clothing (and NO COTTON on you) you can cope with -10 deg to 0 deg just fine, it really isn't a too big a deal. There is no way to cope with 100-110 deg heat, you are going to slow down no matter how you slice it.
one difference is that with extreme cold if you start to have a bad day or have to go to the bathroom or you just blow up during a long run or something, you better be able to keep jogging or take shelter, since if you are stuck outside and it is -30 or worse (even if it is wind chill) you could die. When it is hot out, if you start to struggle (not with heat exhaustion, just having a bad day) you won't die, you just need to stop and walk. You aren't gonna die from exposure.
I have done long runs in rural Michigan and rural Wisconsin and if the cold was really severe, I did 2 x 8 mile loops so I could stay on courses that had a house every half mile or so. The other courses had long, open stretches with no wind break and a house every mile or so, but you never knew whether someone was home or not.
I did this when I was young and stupid and didn't have adequate face protection. Now I have a ton of winter clothing.