.357 is certainly different than 9mm. It is. No, disagreement there. It is rimmed. It can put out more ft/lbs of energy.
It is just not different enough. One of the biggest arguments that people have for buying a .357 is the type of gun that you "get to" shoot with it: a revolver. And in that arena, the .347 offers little advantage over 9mm. 9mm snub noses? Check. 9mm mid-sized DA revolvers. Check. 9mm full size race revolvers complete with porting/comp. Check. 9mm single action only. Check. Yes, you can buy more types of .357/.38 than any other type of revolver, so if you are a revolver nut, .357/.38 is a must. But, if you only want an example of each major type of revolver, 9mm does it.
.357 would be so much more appealing if it were the same bullet diameter as 9mm. Then, I'd build a 8-shot .357, cut it for moon clips for 9mm and be done with it. But .38 special is almost as expensive and as .45 ACP, which makes cheap .38 almost as expensive as cheap .357...
But on average, .357 doesn't really do that much more than 9mm. On average, .357 is pushing around 500 ft/lbs of energy. On average, 9mm is pushing 300+ ft/lbs of energy. The most common high-power 9mm is pushing around 400+ ft/lbs with a high end of 500 ft/lbs of energy. .357 stops at ~700 ft/lbs of energy. So in general, most .357 are pretty close to 9mm, .40 S&W, and .45 ACP in terms of ft/lbs, enough that they are all near the same range of energy.
When you are talking about these high-ends of .357, that's where the price really jumps up to a $1 a bullet or more. You can still get hot 9mm at less than $0.50 per at the 400 ft/lb range. Almost all .357 defensive rounds (not hunting) are in the 500 to 600 ft/lb range, and this is for premium bullets. So, mostly .357 is x2 as expensive for only 200 fl/lbs more energy on average.
Cheap .357 is about $0.50 a round and is around the ~540 ft/lb mark, which puts it into hot 9mm territory.
10mm "wins" this equation with the Underwood offerings. Cheap 10mm FMJ is up in the 700+ ft/lbs range at $0.60 per. Defensive 10mm falls more in the category of .357 range: 550 to 600 ft/lbs. But 10mm is by far less available than .357, with no in-store offerings locally for me at decent prices.
I feel like I should investigate .44 magnum more, since the .44 mag looks like it is widely available. .44 mag is significantly different than 9mm, hovering in low ft/lbs centerfire rifle territory. I've discounted the .44 mag for a couple of reasons: no cheap version of .44 is available to shoot. .44 specials are $0.60 a bullet (closer to $1 per) are not very available. But if .44 mags are $0.60 a bullet at good prices and more available, it might be worth it to have one .44 magnum handgun. One thing makes me hesitate beyond economics on the .44 mag though: the Alaskan State hunting manual recommends that .44 mag and lower are not suitable for brown bear hunting. It is a small quibble, because I have rifles that are suitable, but it sticks in my craw none-the-less.
.44 mag might work as a back-packing gun, too, as I've recently come to learn, in the form of a S&W 329 PD at a low, low 25.1 oz!?! That beats the Glock 20/40 by about 3 oz! My only hesitation is that it probably is really unfun to shoot. It if isn't... maybe I've found a good back-packing gun? My inclination in .44 mag, because I have this wacky porting-sickness, is the S&W Stealth hunter at 56 oz, which seems much more pleasant to shoot.
.44 mag brings me back to .45 ACP/.454/.460 revolver builds. That would give me cheap, available plinking ammo in the form of .45 ACP and on the high-end, I'd have cartridges capable of taking most game in North America.
As you can see, I've probably over-thought this issues, and that is why I am turning to my support group: TFL.