Actually Stephen adds to the point by noting that there are parallels to Q1 and Q2 in Thomas. The interaction between these three creates the document we see before us, with the inevitable parallels to diverse material. I will have to admit to leaning towards the second option here. But the same principle is difficult to apply in relation to the discussion of oral tradition in the first two century of our era.
Oral traditions are by their very nature lost except in so far as they are crystallized in texts.
In such circumstances we need to think carefully not about what is the simplest model, but about what is the most plausible model. It sounds to me, at my present state of knowledge, like the second option above is the more plausible model unless we are radically to revise ideas about the diversity of Christian origins. Since beginning reading on Thomas, I keep stumbling over the citations of Stevan Davies as a major authority on the issue. The analogy with Papias may be a useful one. Here we have a figure who knows at least Matthew and Mark but who says he prefers oral tradition.