IMHO.

I would do both, for fear of cross-contamination. If the fluid is flushed at
regular intervals the hydraulics will thank you with a very long life. Not to mention that as the fluid becomes old the boiling point is reduced severly. It won't affect the brakes in average driving, but it may boil when you need your brakes the most, causing the pedal to sink to the floor as your heart skips a few beats. Plus those expensive abs components don't loike dirty brake fluid. Just my .02.