Well... I can only speak for the futurehosting experience. I have been with them for 5 years, and I can say without reservation that you will not experience a better host.... maybe a comparable one, but not a better one.
I can say that knownhost was my 2nd choice. Anyone I spoke with over there when I was shopping were 100% professional.
You definitely cannot go wrong with either one of them... and that is fact!
How intensive is this one site? Does location matter to you, because each of those companies have different locations to offer. If you are ok with either of the plans they offer, I would take to the forums or Google and dig a little for some recent reviews of both companies to get a better feel for what they are good at and what they may lack.
Then send both a series of similar pre-sales questions and see how they respond.
I think after that you wont need anyone's advices from here and you will have a good idea of who you want to go with at that point. Best of luck!