Looking to stock up on carry and home defense ammo. I have never shot either round through my pistols.

Carry = S&W Shield 9mm

Home Defense = Glock 17

Do I want Federal HST 147g P9HST or Federal HST 147g P9HST4 (+P). The price is almost the same.

Thanks for your input.

Velocity helps beat hard barriers.
Federal has reduced the velocity of both rounds by 50fps at the same time they shortened the skivving, leading to reduced expansion, in response to some instances of jacket separation in auto glass tests.

The +P is the new standard P in effect and the expansion is still less than the old standard P while gaining some penetration.  Expansion was always HST's strong suit so it's still a good round, but it's a bummer to see it give up that crazy big, sharp-faced mushroom.

In bare gel the +P still looks impressive, but in bad guys bullets tend to end up more like heavy clothing test bullets.

It was never a snappy round.  Get the +P, no question.

When did they change the ammo.   Is the same true of 124 gr rounds as well?

I feel the P9HST2 will do you an excellent job. Without a doubt it's going to perform fine for you and
your needs. I'm not a fan of any of the +P offerings for pistols. Either way you choose you can't go wrong, HST is a great round and the new verses old HST is splitting hairs in self defense.

You cannot shoot +P per the manual from the Shield. So there is your answer.

Not a smartass but does any pistol explicitly say you CAN shoot +P?  Ive shot 147+P HST back to back with std 147 HST in my Shield, and it doesn't feel like much of a difference to me.

It voids the warranty. The rest of my pistols have manuals that state it's okay. I'm pretty sure that two mags of it will not kill the gun but if it prematurely makes the springs or other parts wear out quicker...well you can always claim that particular standard pressure stuff lot was really hot

I'm using Micro 150 HST out of my Shield. Standard pressure and +P differences are so benign from that short of a length of barrel that it really doesn't make a difference.

Sig P320 explicitly says SAAMI spec which includes +P. They do not impact warranty status. They even issue guidance that +P ammo may wear the springs faster so the regular service life is reduced and you should change them sooner.

From what I've read, the 3.1" barrel in the shield isn't long enough to burn and take advantage of the additional powder of a +p and is a waste.

More flash too.

I always choose standard pressure for these reasons.

Faster follow up shots with reduced recoil
Subsonic so it is slightly quieter (147 std pressure)
Less wear if you shoot it a lot
Ballistically they are so similar why increase the recoil and some loads the +P does not penetrate as well and standard pressure in some scenarios not all but really it is splitting hairs

Standard pressure for me if I had to choose.