If that's what the original poster meant by "species" -- then, yes, you are correct; they are both Maia (i.e. a lower order of angel). They would be the same "species" in the same sense that St. Michael and Lucifer are the same "species". But there is no question that evil is only a corruption of good -- a poor imitation borne out rebellion and pride.
I jumped the gun in my "They're not" comment because I thought someone was implying that good and evil were equal forces in JRRT's work.
It should be be remembered that Tolkien himself saw The Lord of the Rings as an essentially Catholic work.
The religious aspect of LOTR is subtle but implicit. JRRT described it many times as a Catholic book, which it is. Nothing in it is out of the scope, so to speak, of Catholic theology. But he was careful not to mention any established religion because he believed that his tale was set in a pre-pagan day, not just a pre-Christian age. It is implicit that there is The One (Eru, Illuvatar), and the Valar who are as the Archangels. But this is also an age that is pre-superstition. When they invoke one of the Valar, they call on a specific one -- just as a Catholic calls on a saint (Elbereth!). Sometimes Aragorn calls on his ancestor ("Elendil") again, just as a Catholic would invoke a saint or an ancient Roman would his ancestor.
Perfect analogy--JRRT is smiling on you!