I have XS Sights on every firearm I own, save one. The pistols, the AR, and the shotgun....all have XS sights.
Big Dots for M&P are standard only (although some 3rd party websites list them as 24x7 express, no M&P sights come with tritium in the rear....there is no room). Lack of tritium in the rear does not affect the effectiveness at night at all. If you get them you'll see. If you can see the full dot and put the dot on the target, you'll hit it.
Sight acquisition is incredibly fast. I am a fan. M&Ps and Big Dots go together like peanut butter and jelly.
Tracy