Lei Shen was harder but Garrosh is taking most guilds far longer to kill just due to the length of the fight. Before our xmas break we were pushing close to Phase 3 and it's a long old fight every time you wipe, kinda rough considering there's another two phases ahead of us.
Also, a lot of Lei Shen's difficulty was in the transition phases, so a lot of your wipes came early on, making for short fights and more attempts. Garrosh's difficulty is from those empowered whirls and that shitty terrace room (on 10 man if you don't have two druids) which both come quite a long way into the fight. It would be nice to have some sort of buff like on Sha of Fear for those longer fights, it is very boring to sit though six or seven minutes of the fight before getting to a point where you start learning it again.