I think someone else pointed out the recoil factor in needing a better sight. My thinking would be to get a more midpriced reflex for your Redhawk because of the recoil. You can probably get away with a cheaper one for your Mark. This way you sight them in and don't have to worry about switching them.
Regarding the low light or dark situations, no the reflex sight itself would not really help that situation. Most reflex sights are only putting that reticle on the glass you are looking through, they are doing nothing for iluminating your target.