I have two M&P pistols that are older PD trade in’s.  The rear sights still glow but the front sights on both need replaced.  Who makes the original NS for the M&P’s?   I generally just buy NS as a set but are they financially cheaper to buy just the fronts?

Just my two cents worth since the rear sights are probably pretty old I would just replace them all.  Ameriglo or XS are good to go I use both.

Are the slides the optic-ready CORE slides or just iron sights?  I think Trijicon used to make the factory nightsights with the sharp corners/ears and the more recent ones appear to be from Truglo with rounded corners.

Just regular slides, Copy thanks!  

I'd replace them both. For whatever reason the older night sights would randomly eject the front sight vial.

Another vote for if you do one, do BOTH.

Bigger_Hammer

I would replace both sights and go with something other than OEM sights.  The OEM sights are very oversized for the dovetails, and S&W obviously uses some manner of machinery to press them into place.  I usually find it takes a vice, a steel punch and a big hammer to remove the factory sights.  They are almost always damaged during this process.  I've heard of some so tight they needed to be cut out.  

Because I wanted to finish an Apex installation I recently re-used a rear sight on a M&P while I'm waiting for my Trijicon HDs to arrive.  The amount of filing I had to to on the OEM sight to get a proper fit so I could drift it back into place was ridiculous.  

I've now done sight installs on 10 M&Ps (some mine, some friends) and I really, really recommend using aftermarket sights.  You still need to do some fitting, but it's more reasonable.

I'd highly recommend Trijicon HD XR sights for a dual-purpose defensive and range gun.  They can usually be found online for around $120 a set.  Make sure to check out the XR front sights that are narrower than regular HD.  The regular HD front sight blade is too wide for most target shooting but works well for defensive distances.