-- you resemble a fundamentalist in not seeming interested in learning about the actual history of the two religions. The fundamentalist will say that all resemblance comes from Mithraists copying Christians. You say the opposite. And both you and the fundy are clearly grinding axes. Excuse me, but some of us would rather study history.
It's not clear which of the two religions is older. It's not clear that either directly borrowed anything at all from the other.
And anyway, it seems to me that an actual atheist wouldn't favor any religions, on account of being... you know -- an atheist. (Yes, I am accusing New Atheists of atheisting improperly.)
If you know me, you know that I'm all about the primary sources. Read modern historians if you want to, but check their work: look at the materials from which they formed their writing, and see for yourself whether you would come to similar conclusions. And yaaay, Maarten Jozef Vermaseren collected the small amount of evidence of Mithraism still existing in the Corpus Inscriptionum et Monumentorum Religionis Mithriacae.