Both are outstanding companies. I personally would have zero hesitation buying either one of them at any point.
Tourmaline is no doubt cheaper. This is why you're likely seeing it as the better buy via our screener and articles around the internet.
If I were to pull the trigger on a buy right now over the short term, I'd likely be adding Tourmaline. I think there is a value disconnect there because of the volatility in natural gas pricing weighing on the company. My strategy there would be to buy, hold, and exit once the value gap is closed.
But if I were to buy one with the intentions of holding it over the long term, I'd probably choose Canadian Natural. The company is, in my opinion, the best oil and gas producer on the planet. A bit more expensive right now, yes, but I think if we look back ten years from now, there will be a good chance that Canadian Natural has provided better returns than Tourmaline.