I have owned the 6D for a while now and it is my favorite camera so far. While some people here expound on the lack of this or shortfall of that, I have found that ALL cameras lack in one way or another. Anyone using any camera learns to deal with what is lacking and work with what is available.
That said, what no one has mentioned here is that there is more to a camera body than its AF point abilities. Ergonomics play a very important role in whether a camera will fit you or not. By renting the system you will have the opportunity to feel how the camera fits in with your style of shooting and if any discrepancies can be overlooked or dealt with.
You will also have the chance to compare the benefits of a full frame camera against what you have done in the past to see, in a real world environment, if the benefits outweigh the cost. I know many photographers who have left full frame cameras behind because their benefits just were not there for them. Many of these people have left dSLR cameras all together for mirrorless, so don't let all this rigamarole about AF points, FF vs CF, low light benefits, etc. sway you from making your own, qualified and empirical decision after you have used it on your own.