If I had to choose, I'd say hot weather is easier on a car's longevity than is cold weather. Here is my reasoning:
- Breaks fluids down faster
- Keep an eye on engine oil/coolant levels.
- Maintain fluid more often (change oil/coolant sooner)
- Tires wear out faster
- Hot asphalt tears up the tread faster (higher temp rated tires are needed here)
- Heat is the mortal enemy of automatic transmissions
- If extreme heat exists, an extra capacity transmission cooler can be added
- Fluids are thicker/stiffer
- Increased warm up time is needed
- In extreme cold whether, the need for a block heater is a must
- Oil doesn't circulate as quickly which can cause increased wear
- Parts become brittle when cold which causes breakage which does not occur in hot climate
- While metal is a big concern here, even rubber can become brittle and experience breakage
There may be other things which I'm missing here, but if you look at what I wrote, most everything which can happen to a vehicle in hotter climates can be adjusted for by doing proper maintenance. As long as that's done, wear should be about normal for the vehicle. On the other hand, there isn't much you can do about the cold. When a vehicle is cold, it will stay cold (talking mainly about suspension parts). All of the cold issues cause wear/tear on the vehicle and there is just about no way of getting around it. Even if you keep your vehicle in a heated garage, it will get cold again once it's outside. Metal fatigues faster because it becomes more brittle and forms stress risers easier. Grease doesn't lubricate as well, so parts experience more wear (ball joints, tie rod ends, etc.).