I alwasy played Kasumi casually while instead Phase 4 is one of the characters that I'm trying to make become a sub, for me they appear two different characters. Also for Kasumi teleport is an element of support for increasing the damage, I think that it become a true element of playstyle only when performed through her holds (that led to a different kind of offense).
On the opposite Phase 4 rely completely on it, so the two ways to fight (and even the visual outcome) of the two characters appear different.
It's possible that is a little difference for many people, but for me it's enough a big one to make me wish to learn one and not the other (and with this I'm not saying that one moveset is better than the other).
Actually I really like Kasumi as character and I love her Tag Team with Phase 4 (my favourite one after Kasumi - Ayane). It's just that about the moveset matter there are other characters that I like more and that I would wish learn before (not only P4, the list is pretty long xD).