Of the two choices below ONLY, which do you think is more difficult, and why?

I think usually it would be father and young daughter(s) for a couple of reasons. 

First, I think that usually the discomfort level of the father would be greater, making the conversation more difficult. Secondly, there are more intricacies to the female side of what's involved with "the talk" that men just can't understand or explain as well because they've never experienced it. Same is true of the reverse, but I don't think the differences are as great or on the same level. 

Father with daughter.

Hard on the father's behalf because he knows what he was like as a young buck and doesn't want his daughter to be subjected to such a beast.
Hard on the daughter because she is afraid what he might say about mom and women before mom and doesn't want to know.

Hard on both of the examples.
When we are in a parent-child relationship, sex, even though most of us know that is how the children came into the picture, don't think about our parents in that manner.  When most think about sex it is on an intimate level of desire and lust.
Hopefully, we don't lust after our parent figures.
I would expect them to be role models, teachers, care-givers, and protectors.  Lusting after their bodies for sexual gratification does not fit into the equation.   

It depends on the relationship the children have with that parent.  If I was to choose I'd say father and daughter, simply because men appear to get embarrassed and do not like the idea of their daughter having sex, and the daughter doesn't like the idea of her father thinking about the fact that she may have sex.  
Women tend to be more relaxed having carried their offspring in their womb for nine months, and having changed their shitty nappies and wiped their arses, coupled with the fact women have had more of their orifices poked and looked at over the years.   

Hello there Randy D
Forgive me for not adhering to the instructions included with this question...

I was/am a single dad of both a boy and a girl, and dealing with the stresses of adolescents was equally as challenging for both.
I never actually had "the talk" with either of my children, never really saw the need. Firstly because they probably knew more than me about biology and the various functions of the body...
Secondly, because I always took the strategy of providing an example of behavior rather than rely on a mere "talk". I tried to show my boy what I thought a man should stand for and my daughter what a good man looks like, and to not settle for less.
We often had discussions of actions and consequences for all things.
They are both on their own now and have respectful,  healthy relationships with their significant others...

i think theyre both hard

Father talking to daughter. 

I never had "the talk" with my girls.  I answered their questions as they came up and eventually everything got covered at the time that was right for them..............since they asked about it.  Not all kids are as willing to try to mortify their mother as mine were.    I always managed to answer truthfully even when I wanted to evaporate.  It was harder for me than for them, trust me.  When it was time for them to start spending time away from me, dating, and going off to college, I knew I had done all I could do because I knew that when they had to make a decision, they would have no doubt as to what would mom say about this.  That really is the realistic benchmark to which parents must reach.  Beyond that, they are on their own.