The only real answer to this is that both are way beyond what any reasonable, experienced person would consider a "minimum" in performance for speed solving. Either get both and decide for yourself, or take a chance and just get one or the other. The risk is not in whether or not it will be the better of the two, it's in whether or not you would prefer the feel of one over the other. I guess one significant factor is what a person does to their cube. There's more than one way to properly lubricate and break in a cube, and likewise there's more than one way to f*** it up. If someone is slow with a decently prepared/maintained/broken in GuHong or Zhanchi, then it's 0% the cube's fault. Especially if their reason is that they're getting pops and lock-ups.