I prefer the flat load bay of the Niro as I have a heavy wheelchair to drag in and out of the trunk. It might have less usable space than the Ioniq, but it's enough for me. If I need more, I can fold the seats down.
Yes, by the numbers the Ioniq has more cargo space. 23.8 cu ft vs the Niros 19.something. The Prius has a tad more than the Ioniq at I think 25.4?
However, I bought the Niro for cargo space, because I think the usable cargo space on the Ioniq and Prius are less. It's angular, so I feel it limits how many boxes/suitcases you can fit. Plus, in the Prius and Ioniq, you very quickly begin blocking the lower part of the rear window (I agree with others who did not like the split rear window in either, although I found it especially limiting in the Ioniq, which I otherwise liked more than the Prius).
I took measurements and mentally did up things on how to fit various suitcases and other things into the Niro vs others and felt that i had a better space with the Niro. Or at least the same but more passenger space. If I didn't have a kid I probably would have ended up with the Ioniq.
If you *really* want space though, the Prius V is the way to go. You lose some more mileage, but get a mini-van of space in a wagon looking thing. Some of it's tech is out of date though (unless you spend through the nose) which is why I didn't go for it.