You're right, one won't work for every situation.
When you say "getting," do you mean buying or renting? Rags are super cheap to rent, so rent a few different selections if you can. I suppose how easily available and how many options you have will depend on where you are located.
If you have to buy just one, the .5 stop difference between 1.25 and .75 isn't going to make or break the shoot. What time of day are you shooting? What kind of climate do you live in—can you expect a lot of harsh sun or is it often overcast? Are you going to be under partial shade or out in the open? Do you have reflectors or HMIs (and a way to power them outdoors)? If you're going to be under hard midday sun and you have tools to create some controlled light on the subjects, go with denser diffusion.
Regardless, make sure you have adequate gear to safely fly a frame above talents' heads outdoors. Use combi stands with at least two sandbags per stand and be prepared to tie the frame off if there is any wind. One gust is all it takes.