It depends on what kind of adaptation you want.

I personally view cardio as just a means to improve my cardiovascular system, not muscles. So I just do boring steady-state (where I can easily rap out a song or sing Bob Dylan's lyrics).

If I want increased mitochondria and the size of the muscle cells, then nothing beats an even more efficient method of strength training. Just 5 minutes of work (1 minute per set) of lifting a heavy weight for 5 times. But even here there's concept of RPE where you don't accumulate as much fatigue as you might think.

I first did strength training and my resting heart beat dropped from 75 to 60 after about 8 months. After that I started doing steady-state cardio of 30 minutes on rest days and it's now at 55 after 3 years.