Personally, I think that the only similarity between the two is that they are both R&B singers. Their dance styles and voices for that matter aren't anything alike, neither is their style so I don't see how one can "replace" the other.
Anyways, there's room for everyone in my opinion. Mya just needs a much better promotion team behind her. Hopefully with this album (Liberation: June 26th) that will occur. As for her new video its Hot! I don't expect anything less from Mya when it comes to dancing and the song is pretty nice too.