If you’re using external desktop HDDs, you don’t need the powered hub as it’s a lot more expensive than the unpowered one. And it could back-feed your Pi also.
The best hub for the Pi is the USB 3.0 hub, which is actually the USB 3.1 Gen 1 hub. Make sure you buy from the manufacturer that discloses their hub’s controller chip. I am using the one with VL817 chip. It’s rock-solid, I have never had a problem with it.
I am not sure whether USB 2.0 hub would be sufficient. However, I don’t see any point in using the 2.0 hubs as the 3.0 hubs (unpowered) are very cheap already. I am using the one from Orico.
You can see the limitation of farming with Pi here.