“Is an MRI or CT scan better for diagnosing a blood disorder?”

Scans generally do not detect or diagnosed blood disorders. There are a few exceptions for this, including some blood disorders that can cause enlarged lymph nodes, or damage of the bones. Sometimes the bone marrow appearance can be the first clue that there is a problem with the blood.
Therefore the choice of scan depends on what kind of blood disorder, and what the clinician is looking for.

Most blood disorders are not detected by imaging but by blood draws and potentially with a bone marrow evaluation. Certain hematologic malignancies can be detected by imaging, but the best modalities differ based on what kind of tissue one is trying to assess and which malignancy one is concerned about.