I think that's gonna be a listener preference decision. Both are very good speakers.
Both are heavily discounted right now so you can get a great deal on either system. I think the RC-70s and Omni 15's are $399 (or $499) each and the centers cost around $299.
Mirage and Energy are sister companies that were Canadian based until Klispch purchased them. But they use different technology for these speakers. Energy uses the standard speaker monopole technology while Mirage uses Omnipolar technology which disperses the sound all over the room.http://www.miragespeakers.com/na-en/about/philosophy/
Even though the speakers are about the same size, the Energy RC-70 weighs about 60lbs, while the OMD-15 is around 35lbs. The build quality on both looks great.
I own several Energy bookshelf speakers and the RC LCR center. Always been curious about the OMD-15 but only seen them online. Don't have a local store that carries them.