Assuming we are talking about nickel white gold's which are the most popular the 18kt White is harder than 14kt white. All suppliers' white gold varies in whiteness and hardness which the two are directly related. In general the whiter the white the more nickel that is used and the harder it becomes.
DHF's 14kt white is closer to 14kt yellow in terms of temper and workability with a grade 1 white color. If choosing 18kt white I would recommend an 18kt Palladium white which is much softer and easier to work with. Here the palladium is used instead of nickel. Slightly more in cost due the palladium content in the alloy but a nice bright grey white and soft.
Hope this helps.