Best choice for home defense

As mentioned, the best home defense gun is a long gun, whether a shotgun or carbine chambered for a pistol cartridge doesn't matter much. A long gun is much easier to learn to shoot well and has a much higher hit ratio than a handgun in typical hands. Their downfall is manueverability in tight places such as going around corners and through doorways. In any event, I would not put a sling on any long gun for use in the home as the sling has a nasty habit of catching on things such as doorknobs, being in the way of loading or plain handling the gun, and there is no real need or use for one, after all, you aren't going to be marching in formation several miles with the gun.
As a home gun, what you have now is probably more than adequate, a 38 Spl snub nose that one practices regularly with stoked with 110 gr hollow points (either +P or not) is a pretty good combination. On the whole, a hollow point bullet is the best choice as a defensive round as it is supposed to expand and deliver more of its energy on the target. A semi-wadcutter is designed for deep penetration which is accomplished by not expanding. It is also designed to cut neater holes in paper targets for ease of scoring than a round nose bullet. The 9mm with 115 or 124 gr hollow points it feeds well is also a very good option and one you currently have.
As for any future purchases, any of the ones you are looking at (45, 40, and 357) would be fine though maybe not substantially "better" than what you currently have from a chambering perspective. Whether 45 ACP, 40 S&W, 357 SIG, or 357 mag, the ammo costs will be pretty comparable, "stopping" ability is very similar, and the choice of caliber mostly coming down to the style of gun you want or vice versa as the 357 is mostly a revolver round while the others are mostly semi-auto rounds. Except for the 357 SIG, I currently have or have used all the ones mentioned above and feel they would make a good choice for your expressed purpose. This doesn't narrow things down but it is the best I can do.

The "FBI Load" for 38 Spl. is 158 gr. LSWC-HP hollowpoint and +P loading.

The nice thing about a .357 mag is that they shoot 38 Spl. It's serious defense, but what you have is serious defense.

I recently got a Springfield XD "Tactical" (5" bbl) 45 ACP. This features the "compact grip" -- two mag lengths/capacities. One is short, 10 rd, and provides a short grip for CCW. The other is longer, 13 rd. and extends the grip length about 1 1/2".

I put a Glock GTL 21 laser on the rail. This makes the gun function nearly like a very compact shotgun. I put the red dot on the target and pull the trigger. Laser allows you to keep your eyes on the target -- AND you can keep the dot on the target without having to line up a sight picture.

Long gun for defense -- That's the preferred option. My nephew just bought a Rem. 870 and uses a cable bore lock to secure it. The lock runs through the ejection port and out the bottom of the load gate.

I'm not a big fan of the 40 Smith. I reload, and it's complicated to reload. Mostly though I think I resent picking up 40 Smith cases on the range, thinking they're 45 ACP.

The design concept on the 40 Smith is to put 45 ACP ballistics in a 9mm frame size gun. Law enforcement was looking for a more potent round than the service 9mm -- which was bouncing off streamlined windshields.

The 40 fills the bill and the 9mm frame size results in a gun that can be gripped by women with smaller hands.

The 40 Smith works, but if you're looking at 40 Smith, you may as well get a 45 ACP and have all the ballistic options of a 45 ACP that the 40 can't meet. 45 ACP provides a larger dia bullet, heavier, and at the upper end of the load range, more energy to the target.

In the nightstand I have a Smith & Wesson Mod. 629, N Frame, 44 mag, stainless. 4" barrel. I can carry it as a side arm. I can "download" or shoot 44 Spl. loads for a faster follow-up shot.

Hands down for home defense it's hard to beat.