Assuming your flash is performing in a 'fill' role, and you're taking about on-camera direct flash, then I'd forget about any diffuser.
It will just scatter the light too much, losing you range and power, and as still a small light source (not much bigger than the flash head) .... As we've seen in your other thread recently even a small softbox that is a distance away from the subject still produces fairly harsh light.
Also a diffuser is just one more thing to mess around with, distract, or fall off ! The main things to concentrate on are the techniques you'll use to meter and cintrol' ambient light and balance that with the flash. It is likely to need a flexible approach outdoors, assuming you'll be moving around a bit.
---------- Post added 08-06-16 at 12:10 ----------
Also, if you are using a daylight white balance then you might want 1/2 CTS gel on the flash to stop the skin tones looking to cool.
I saw the previous post about using the GF Lightsphere ... Now that's one diffuser that certainly won't fall off, that's fair to say! I think it will scatter the light though and not make a big difference to the light quality, especially from any distance.
---------- Post added 08-06-16 at 12:46 ----------
The Lightsphere works well indoors though like in Noels shot with the Registrar there ... It spreads light all around 'dome' fashion, and you get a soft and wide coverage. Is not particularly directional though .... I'm just not so sure about it outdoors ....I remember some heated debate on here a while back! I'm more convinced by the "big is best" argument, and unless you're very close then most small diffusers will still be a small light source.
Last edited by mcgregni; 06-08-2016 at 05:39 AM.