OOC: They are not completely different, just slightly different. Negative Dimensional Energy generally attracts Negative Energy of the body/soul. Meaning, negative dimensional energy can cause negis to come forth. But then, controlling negative energy wouldn't give you control over a negi, as a negi is simply born from negative energy.
Orpheus does have the ability to cause negi to get stronger, but not the ability to make them weaker. He can add negative dimensional energy to an area and intensify it in a concentrated area, thus strengthening a negi, but taking away negative energy wouldn't do anything unless the negi is just at that time coming to life.