In short self defense-oriented barrels, such as the standard S&W Shield, I prefer the lower end of normal bullet weights, 9mm 115 grn but not lower. Everything shoots fine in my Shields, so it is not a question of reliable functioning, which is always the second question. The first is, does it always reliably and safely go bang.
I also shoot 115 grn self defense loads, Barnes TAC-X PD. Because they are all copper, they are the length and diameter of 124 grn bullets, just lighter because of having no lead. The longer hollow point bullet gives better terminal performance than shorter ones since there is greater length to the bullet to peel back while still retaining the base for weight and penetration.
Heavier bullets leave short barrels at reduced velocities. Reduced velocities generally give poorer terminal performance in self defense loads, unless the bullet is specifically designed to open under those reduced velocities. Heavier bullets have slightly more recoil, typically. In a small gun this can be a bit of a problem.
When you go to AmmoSeek.com you will find 9mm listed from least to most expensive (be sure to add shipping and tax if applicable). The first dozen or more loading are always 115 grn target ammo. It's a smaller bullet so it's cheaper to make. It's a shorter bullet so there's more room in the case for powder, giving loaders more choices. It is arguably the most prolific bullet weight for 9mm and is therefore cheaper to manufacture because of volume costs, especially for target ammo.
124 grn prices can rival 115 grn, but usually it is a bit more expensive and even a bit harder to find in crunch times like now.
Other alternatives for lower priced range ammo include buying steel cased ammo, some times with copper plated steel bullets, and "R", Reman, or remanufactured brass loadings. The latter use previously fired brass just like a reloader at home would, so that makes them a little cheaper. Typically the other components (primer, powder, bullet) are the same quality as all new ammo. These two options will save you money, but not much.
Many feel steel cases are hard on a gun compared to more malleable brass cases. Many argue that "R" loads are made specifically for low prices, so quality sometimes suffers with smaller outfits doing the reloading without the sophisticated machinery and quality control of brand name manufacturers.
In any case, the prices for 9mm target ammo are falling, and $0.50/rnd today (10.28.21) is price gouging. There is an array of loadings available, especially in 115 grn, that are in the $0.32-39 range delivered. When I see anyone selling 9mm for over a net of $0.40/rnd today, I keep walking.
