Some thoughts:
In my opinion, unobstructed and larger fields of view trump all other considerations. The point of the dot is speed of target acquisition and anything that hinders that is counterproductive.
The higher end optics rarely fail. Between tritium, high battery life, or prism etching, getting illumination or keeping a reticle in virtually all conditions make the optics that much more reliable. The need for backup irons is minimal. (Honestly, In a CQB 9mm PDW ranges, you could probably hit a man at 100 yards with no optics at all, just looking down the rail...)
Co witness is among the worst of all options in practice. You get all of the clutter from the irons and the optics edges and controls in your sight pucture all the time. You don't get much of the speed advantage because of it. In fact, most people would probably be better served with a tritium front sight post on their irons over a cowitness dot.
In practice, simply running a high quality optic, low with no irons, or above the irons, is great. If you are ready worried about needing those irons, use a quick deteach mount for the optic so you can shed it and use the irons in a hurry. Give the optic its best chance to succeed, and don't worry so much about the backup.
Just my two cents.
