I should have said "Climbing Mountains" in the title. Many of the people I know who say the wind is harder than hills have never climbed a mountain on a road bike. There is a difference between a hill (4 or 5 miles long and a mountain 50-70 miles long.
Which is harder for you? I know many people who say the wind. Myself I say the climbing. I've done a few centuries with 10,000 ft of gain and climbing straight up for 70 miles is more taxing on me than a century with wind.
The centuries I have done with wind can be tough but usually the wind is only one direction. So after an hour or two, there is sure to be a change. With a 70 mile 10,000 ft climb, it is sure to be a 6 hour struggle.
Plus I do very well into the wind. Maybe because I don't shy away from it. If the forecast says 40 to 50 mph winds, I'm still going out there to ride. Most riders will stay home and the trail will be empty. Even on a day with 15- 20 mph winds, you'll be sure to see a line of riders on my wheel. Singles or tandem! 😄