I would say landing on the ceiling would be easier. With landing on the ceiling all you do is just fly inverted upward.
Trying to land on the wall you can't hover staying at 90 degrees, parallel with the wall. It would be much harder to land on the wall I would say.
This.
When you land on the ceiling you just have to manage to flip the helicopter upside down, then slowly push it against the ceiling. With going against the wall, you have to turn it at just the right angle and very quickly, before it falls or turns, having no room for error.
Also:
I felt inspired.
So I tried this in GTA... LOL
It went well imo, seeing as the helicopter just slid off the building and landed on some innocent civilians and a hotdog stand.