If you want a diffused look, it just depends on the look you want.
It sounds like you want the diffusion to also act as a contrast-lowering filter, so I recommend a "mist" diffusion like Tiffen ProMist (regular or black), Schneider Frost (white or black), Tiffen GlimmerGlass, or Schneider Hollywood Black Magic (combination of a 1/8 Black Frost with degrees of HD Classic Soft) or Schneider Classic Soft Blacks (combination of 1/8 Black Frost with degrees of regular Classic Softs.) Think of a Tiffen GlimmerGlass as a Black ProMist except instead of black specks you have silver specks.
Classic Softs have a large pattern of "lenslets" (sort of dimples in the glass) that can produce rings around some lights or an enlarged out of focus blurred fringe around bright edges on longer lenses, and the pattern can come into focus if you stop down too much (hence the finer, smaller pattern in HD Classic Softs & Black Magics).
Filters with black specks in them like Black Frost or Black ProMist can also have issues if you stop down too much and have the patterns come into focus (looks like the lens has dust on it if you stop down too much) though you'd have to stop down quite a bit compared to the pattern in Classic Softs, which is larger and easier to see.
What's nice about Schneider Hollywood Black Magics is that the mist element is constant (1/8 Black Frost) but you can vary the degree of softening if you want to do that, as opposed to using a 1/4 Black ProMist or a 1 GlimmerGlass on everything, which also works but it's the same strength on close-ups and wide shots. And if you jump to higher and lower levels of ProMist or GlimmerGlass, you also change the amount of halation and contrast loss, so there is less matching in look compared to doing a wide shot with a 1/8 Hollywood Black Magic and a close-up with a 1/4. And since the base of the Hollywood Black Magic is a 1/8 Black Frost, you can also switch to just a 1/8 Black Frost if you want something even lighter than a 1/8 Hollywood Black Magic.