In my view... when it comes to defensive handguns, The front sight IS the red dot... If you know what I mean.
I think it would take less time to find your front sight on a target, then to learn to keep a small window containing a red dot in constant view...
Also, if your handgun needs to be in effect yet is not able to be brought to eye level in line with the target, you will not see the red dot, but you might still be taking the time to think about finding the dot... at which point, it may be too late.
Standard Iron sights can be seen consistently, and you can learn to point them, and then bring them up to increasing degrees of accuracy, until you have a normal sight picture as if you were shooting a rifle at long range.
Thats why I think Night sights are so valuable.
I think it is a different story with a rifle, since the gun is normally fired from a "supported" position on your shoulder. Keeping the red dot in line with your sight.
My biggest reason for Iron sights only is that I dont want to have 2 sighting systems to think about... It would require me to make an extra decision, to decide if I was going to use one system or another depending on the confrontation. Generally a self defense handgun will be used at close range, and a smooth transition from "point" shooting to "index" shooting to "front sight shooting" to "dual sight aimed shooting" is a more natural progression than changing from Irons to a red dot depending on the vertical level of your gun in relation to your eye's. In my opinion of course
But there is no doubt... a red dot is has advantages, it will allow for faster accurate shots made at medium to long range.
When it comes down to it... what ever you feel the most comfortable shooting is probably what you'll be best at.