Thanks everyone for the answers.
I was used to 170mm cranks and high cadences but sometimes I felt that it was too short that I can rev much too fast (BTW that was a MTB).
I bought the 175mm, put it on my sloping geometry road bike and guess what?? I feel really comfortable. I don't really feel any difference at all. My saddle is at 67cm, which is just 1cm below my standart. I'm planning to raise it anyway...
Again, thanks.