Both can turtle and space well. Really good close in with 66p and both really rely on 6p mix ups. I don't see how they differ. But then again it's my perspective. If hayate had her 3KP 8 mixup they would be pretty much the same. Momiji just feels like a better hayate.
Every ninja has good spacing capability, it is not limited to these two only. And spacing is only as good as the player's capability. Momiji does not rely on 6P mixups, the average and novice player will utilize 6PKK as a thing when it's actually pretty medicore compared to everything else in her arsenal for mix up.
Momiji has a better CQC neutral overall and one of the highest damaging i12 frame mids out there, 7K and it cannot be punished on block. She has two i12 frame high kicks, an i10 frame jab, i12 frame 2P low jab and two i12 frame mids, 6P and 7K.
Hayate's CQC neutral is literally just an i10 frame jab, i12 frame 6P mid, and an i14 2P low jab. Both have crushes but she has an instant high crush in her 8P and launches on counter hit.