landing on a comet is more important than landing on an asteroid?

This is a very brief answer but I hope it helps.

Distance

Hayabusa collected samples from a near earth asteroid while Philae landed on a comet after travelling a distance of over 6.4 billion kilometers.

The Type Of Mission

Hayabusa's did not literally land on the asteroid. It just touched down and collected samples. While, Philae had to land, steady itself, drill, study the composition and drill. Plus, the asteroid on which Hayabusa landed was a near earth asteroid and so it's nature could have been known easily. On the other hand, the Rosetta team came to know about the exact nature of 67P just weeks before the landing.

The Complexity

The Rosetta mission used about 6 gravity assists and performed 2 flybys. It was not an easy task to plan a journey of 10 years. Plus, when the probe reaches the comet, radio signals would take a lot of time to reach it and thus it would be very difficult to control the probe if something went wrong. Plus, the surface of an asteroid is a lot very different form the surface of a comet. The Hayabusa Mission was not the first to touchdown on an asteroid. It had been done once by NEAR Shoemaker and so we knew what to expect. See this for a detailed report.

I think this sums up how the Rosetta mission was a major breakthrough.