Range, I shoot both but usually stick with split. Gaps with 3-under would be tighter than the same gaps with split - assuming you anchor with the same finger with both. Anchoring with the index finger with 3-under, the gap would be pretty close to the same as shooting split but anchoring with the middle finger. (But that's getting a little bit into the theory of face walking, a whole 'nuther aiming method.)

When it comes to split finger vs 3-under, I don't think one is any better or worse than the other. To paraphrase what Chad said, you should use whichever feels most natural to you and gives you the most confidence.