This is such an apples to oranges question. If you consider the actual physical activity only, the two sports/activities are so different. Downhill skiing has so much more variation because of the freedom to pick your mountain, conditions, slope, line (unless racing and even then some), and your playing field (resort groomers, resort off piste, sidecountry, backcountry, snow cat, heli, racing, bumps, free ride, park). You can vary your speed, turn shape, intensity, duration without stopping. It's so much work on my quads and low back. Water skiing is an event sport - weather conditions vary, lake shape and water vary, but the course is the course whether slalom or jump and trick is defined by points and time. Slalom is so much work on my shoulders and back.
I've been asked this question for 50 years because I've been doing both for longer than that. I can never answer very well because I love them both. I love the difference in seasons and weather. I love cold, stormy, blowing, powder or wind-buff days on snow and I love sunny, hot, windless days on the water. I also love hot, slushy-bumps spring days, rock hard, hasn't-snowed-in-3-weeks days, and everything in between on snow. I love Squaw Valley like it's home. I also love the challenge of windy, head-tail days at the lake and our crew of members at Bell Acqua. If I had a gun to my head, I don't know, I might get shot.