WSF for 9mm. It's just about perfect for that cartridge. Meters well, burns clean, about the right volume for that casing, yields good accuracy. I've loaded & shot thousands of 9mm rounds, most with WSF. I've tried just about every powder suitable for that cartridge and I always came back to WSF. In fact, it's all I use now for 9mm. And no, I don't work for Winchester or Olin Corporation.
45ACP seems to work fine with more options. WSF works as does WST. I've also used 231. Back when WAP was available (Ramshot Silhouette) it worked well at high pressures but was expensive. If I was going to only keep 1 type of powder on hand for those two cartridges I'd go with WSF because of it performance in 9mm. But why limit yourself to just type of powder?