Do you find it more difficult to negotiate a price with a man or woman? And Why?

More often gender dosent play a key role during negotation, factors such as market condition, pricing, availability, understanding his/her concern etc plays a major role

Depends who is negotiating with whom, I wouldn't recommend man-man or woman-woman negotiations. I prefer cross gender collaboration, it has overall pleasant results. 

I loved your question.  

It is the same to me.  Every one of them has his/her own powers to use to get what they want. in a good manner.

Irrespective of the gender it is your experience and you abilties in making a customer belive on your price that makes a difference.

"Men are from Mars & women are from Venus". If the woman is good at negotiations & and can easiliy digest the pros & cons of the associated tactics I think it does not matter with whom you are negotiating with.

For me gender is not really an issue, it comes to down to how persistent a bargainer can be, some of them are very persistent and won't budge from their price and many other bargainers are just looking to get some % off the price. Emotional bargainers are generally more harder to deal with and I once spoke to a women clothing shop owner and according to him it was hard to deal with women who made him their "brother" to get a discount :)