Choosing between primary advisors for a PhD program I currently work as a research assistant for Prof. A on a government funded project which ends this year. I have advised Prof. A that I am interested in pursuing a PhD and that I would like to stay at the institute I am currently at and to potentially complete it with them as well.

My job has ended up being that I do the bulk of my experimental work in Prof. B's group, as they have the necessary tools and expertise to answer the questions we have and I have established a healthy collaboration. As Prof. B's group is more focussed on work closely pertaining to mine, I find myself more engaged in meetings and conversations of the subject matter with this group than of my own with Prof. A.

Prof B. has since offered me to be a PhD student in their group next year, quite insistently, with 60% primary supervision to Prof. B and 40% supervision to Prof. A. Understandably, Prof. A would prefer it be the other way around.

I enjoy working for Prof. A a lot, but can also see working for Prof. B being highly rewarding albeit a little bit tougher on the expectation and 'cracking the whip' front. Both would offer me an excellent PhD opportunity, but with two very distinct outcomes and professional training. I'm a little stuck on how to decide between them. 

Is it a better option for me to have Prof. B as my primary supervisor given that's where my professional goals lie, or is it more important to work for someone that I am much more comfortable with and have extensive prior experience with such as Prof. A?
This paper asks about postdocs, but I suspect the conclusions would be the same:
https://f1000research.com/articles/6-1642/v2

After surveying Postdocs, they find that the only singificant predictor of life satisfaction is having how collegial atmosphere in the lab is.  

Thus I'd say pick the supervisor who's group you feel most at home in. Your PhD will inevitably suck at many times, and the only people who will really understand what you are going through are those in your group. Make sure they are people you can trust and who will be there for you.