They're very different sports, demanding in different ways.
F1 is much more physically demanding, you have much higher G forces, a race lasts from one and a half to two hours of non stop driving, where as a rally stage lasts a few minutes, you drive at plus 40 or in the rain at 15 degrees, you go up to 330 km/h, plus there are 20+ guys out there with you.
On comparison, in rally you have narrower roads, more dangers (obstacles you can hit), various types of surfaces, but you drive alone, dont have to worry about other drivers, you're in a closed car, the g forces are lower...
All in all, I would say that F1 is more demanding, both physically and mentally, where you have to be on top concentration for two hours. And you can see these by the relative ease that F1 drivers can jump into a rally car with little or no preparation, where as, on the other hand, rally drivers would have more trouble and require more effort to get into a F1 car.
So I would say that F1 is the top, the pinnacle of motorsport and is more demanding, but rally is more dangerous, as can be seen from the number of deaths and serious injuries in the past few years. F1 has done a lot to become safe as it is, and therefore there has not been a single death since the two deaths (Senna and Ratzenberger) on the same weekend in 1994.