I personally think that faster greens, 9-11 are easier to putt on. When they are really slow <8, there are a lot of irregularities and you have to make longer strokes, which is slightly less accurate. I have only played on 12 twice. After a few holes it became easier to putt, but you had to really watch your speed on downhill putts.
Most courses I play are 8 to 9 though.