Warpriest seems slightly more effective in certain areas, but multiclass actually has certain advantages as well and it's a tradeoff--and if the warpriest is supposed to accomplish what the multiclass can't, it needs to be more than JUST a tradeoff.
I think this is a big part of the problem that people are having with these new classes. Are they there to allow players with a particular concept to roll a character without worrying about multi-classing, or are they there to provide something which would be beyond a multi-class character?
If the aim is to provide simplicity for those who want a single class character, I definitely feel Warpriest is more of a hassle to deal with than a couple of levels of fighter. Too many things to keep track of.