Ordinarily I would say take Math2, but you don’t want to take a test when you don’t feel you know the material well enough. Any colleges you are considering that still look at subject tests will want to see top scores. Go for Bio. It won’t look strange. If anything, it will be impressive if you can get top scores on two tests, given all the uncertainty.
As for study materials, I always recommend starting with any official material provided by College Board. There are other prep guides, but only CB materials will have official practice tests, and those are invaluable.