Any gun is enough, until it isn't. That applies to the 5-shot snub-gun, as well as a 30-shot sub-gun. Even an FN-FAL will not be "enough," some of the time. The training issue applies across all weapons.
A 5-shot snub-gun is going to be "enough" for most situations encounted in civilian self-defense, if the defender is prepared to do his part. A service/duty-type handgun allows the defender an increased number of options. A service handgun with a double-column magazine allows the defender still more options. A rifle, especially with a magnifying optic, allows still more options. As the presumed defender, I must decide what is important.
As my aging eyes become a larger factor, two things become important: Natural pointability, in my hands, and good sights. A properly-set-up S&W J-Snub, or SP101, offers me that. Some service/duty pistols do not point well, and/or have sights that are largely useless. In the case of the SP101, the factory grip is just about perfect for my hand, and the stock front sight a quite decent one, so the only remaining improvement might be a Trijicon front sight. A bit more barrel length makes things better, still; my 3-1/8" SP101, fitted with a spurless hammer, may be my perfect/sweet-spot compact, ambidextrous, defensive handgun, that I can truly hide under my clothing, on my slender frame, without having to "dress around the gun."
I would rather have two SP101 snub-guns than just one. More ammo is a relatively minor reason. Accesssibility is more important. (With revolvers, I do not have a naturally "weak" hand*.) From 2002 to 2006, my usual off-the-clock carry ensemble was a pair of SP101 snubbies, and a stout blade, in a big city, in the southern USA, not known for being "safe." This was when my on-the-clock duty pistols were a G22, and then a P229R.
I should note that I have nothing against J-Snubs, or the people who prefer them. In my hands, however, a J-Snub's frame concentrates recoil on the base joint of my thumb, unless I use over-sized grips that cover that part of the frame. Over-sized grips make a J-Snub into a much larger overall package, and the SP101 grip is perfection in my hands, anyway, so a J-Snub is simply not a best choice for me.
Having said all of that, I have tended to dress around double-column-magazine auto pistols, most days, since 2006. With retirement now not far over the horizon, however, and age already affecting some of my physical capabilities**, a weapon allowing me to "take the battle to the enemy" is becoming less important than having a weapon to "scrape an enemy off of me." 2017 may well be the year I return to revolvers for most of my handgunning.
*I write lefty, and throw rightie. Many handguns are close to neutral.
**Minor nerve damage, in one hand, complicates running a slide, and some other aspects of manipulating autos.