As you may well be aware, nothing will be pure white unless it's lit properly. And, conversely, anything can be pure white if it's lit properly.
That being said, if you're going for the whitest white with the least effort, #66 is absolutely chock full of brighteners and will be a little easier to work with (as far as being white, anyway). Its brightness can be a liability with regard to spilling back, though.
I'd rather haul a white board than Ps a tattered, scuffed seamless any day. If this is a fairly high volume gig you'll regret not using it. Especially if you're dealing with subjects that aren't used to being very careful walking on paper.
My preference would be the Savage White #66. Take note -- if you're shooting full length, especially in a carpeted location, the first woman wearing heels will poke holes in your paper. Happened to me once and the only solution is either cropping 3/4 or cloth/muslin backgrounds.
... what's the driving reason for going to paper from muslin?