Depends on airline and class of service.
Since there's only two airlines flying the both the 747-8 and the A380, here's how I see this:
Korean has the same "classic" layout on its 747-8 that BA used to have on its 747-400s - First Class at the front of the lower deck. Sitting there in row1 A/J one has a spectacular view thanks to the four windows per seat and the curvature of the walls which allow an almost forward view. And the business class cabin on the upper deck (just 22 suite-type seats) feels a bit like traveling on a private jet. where their A380 has a huge cabin with an older, pretty dated seat product. So 747 wins when in a premium cabin.
When in Economy Class, though - Korean's A380 has the most spacious economy seating out there, and I'd go out of my way to fly on one of those if relegated to the cheap seats. So 747 when in F or J, A380 when in Y.
With LH, I'd want to fly on the 747-8 only if I could sit in Row 1 of First Class. All other seating options, especially the upper deck business class cabin are nothing to write home about and decidedly subpar.
Here as well, if I had to sit in Economy, it's A 380 all the way - especially if I could snag a (window) seat in the small cabin at the rear of the upper deck.

I'd fly Air China on one of their 747-8s in business class - Air China's 747-8s offer the most (financially) accessible seat "right at the front of the plane" with their small business class cabin in the nose of their 748 lower deck. For me, that' cheap Row1 experience would be the only reason to fly that airline.