There's no difference between the two in respect of Eucharistic transubstantiation; hence answering "the spiritual", "the natural", or "both" are equally meaningless.
My answer to a question on the Catholic understanding of the nature of transubstantiation is supposed to make it clear that what is changed in transubstantiation is the substance of the bread; that is (more or less), what makes the bread bread. The material takes on the substance of the body of Christ; and it is in this sense that we say that the bread and wine become the body and blood of Christ.
But the substance of the natural body and the substance of the glorified or spiritual body are the same. St. Thomas Aquinas, in discussing the nature of the resurrected body, raises the question, "Whether in the resurrection the soul will be reunited to the same identical body?" (Supplement to the Third Part, Question 79, Article 1). He concludes that
The soul rising again and the soul living in this world differ, not in essence but in respect of glory and misery, which is an accidental difference. Hence it follows that the body in rising again differs, not in identity, but in condition, so that a difference of bodies corresponds proportionally to the difference of souls.
(Reply to Objection 2)
In other words, he's saying, the resurrected body (the "spiritual body") is different in appearance to the "natural body": it has different characteristics and qualities (for example, it can no longer suffer injuries). However, he argues, these different characteristics are accidents: things that just happen to be true of a person's body. They don't alter the fact that this is the person's body. That is, they don't make it a different body in substance.
Therefore Christ's body, whether the natural body he had before his resurrection or the glorified/spiritual body he had afterwards, was one body in substance; and thus there is no meaning to the question: the substance of his body is not different, and there's no distinction between the two in the change of substance that takes place at the consecration of the Eucharist.