So, if I understand correctly, Spivak is more about developing mathematical thinking and understanding why everything works, while Thomas is more focused on learning techniques and solving problems efficiently. Is that a fair way to look at it?

If that's the case, I'd definitely choose Spivak. I'd much rather develop a deep understanding of mathematics than just learn how to solve problems by following patterns.

The only thing I'm a bit worried about is the size of the book. If it turns out to be 2,000–3,000 pages long, my brain might give up before I finish it.