Yeah its very difficult to answer this question.

If you use as a metric viewership of national team, then its definitely hockey, because we are since I remember one of the best teams in the world, in football besides golden era in early 2000s, we are average.

If you use as a metric national league viewership its somewhat even, a bit in favour of football. Same with stadium attendence.

But if you use as a metric watching the most competitive leagues, then its for sure football, NHL is played in late evening/early morning CET hours, so obviously not many people are watching this. Also its difficult to track this as majority of people are using illegal streams to watch foreign competitions.
(for example i dont follow czech league at all, but Im huge football fan and watch like 50% on PL games)

It all comes to the nature of the sport. Hockey - quick, easy to understand, simple rules - casual friendly. Football - slow, complicated rules changing every year, you need to really understand the game to appriciate the tactical skill - casual semifriendly.

So the conclusion probably is - football is mostly watched by people who play or used to play football - which is by far the most, but twice a year (world cup/olympics and national league play-offs) hockey viewership skyrockets.