Just remembered about dot optics.
The way the red/green dots look to me says I have astigmatism. No matter how I twist/turn my head the 2 MOA Holosuns are squirrely looking against a target. Not a dot, just squiggle/twirly starbursts.
But the 6 MOA K series (also smaller and a better match to the width of the P01/PCR/even CZ75 slides) Holosuns have a bigger and much nicer, rounder, more focused dot. Just something to consider. When I get my M&P Shield .40 S&W slide milled for a dot it'll likely be the K series Holosun. I'm one of those guys who doesn't care about the window size because I'm not looking at the window, just where the dot is on the target.