Andy-
  I would suggest that you use MRIcroGL to view scalar values like FA and MD. MRIcroGL is a volume renderer, so it is great when you want to visualize how these values differ across different locations in the brain. On the other hand, Surf Ice is a surface renderer, which attempts to show what the surface of an object looks like, not its interior volume. Surf Ice is good for showing tracts (streamlines inside a volume) and values that differ across the surface of a brain (e.g. gray matter depth or statistical significance for a functional imaging task). On the other hand, we expect MD/FA to differ between the surface and internal structures.