I would prefer this feature on a file-by-file basis. I usually prefer the unified diff, but for some changes, the diff is so bad that it becomes useless; that's where I would use a split diff.

Another diff-related improvement I'd love to see is useful whitespace removal. I know you can do it with a URL flag (`&w=1`), but then you can't make comments, meaning you have to switch back and forth to use it.