Some good definitions there.

There are several factors that affect how a plane stalls, if it has a strong tendancy to "snap" or not. Three of the biggest are the wingloading, the airfoil, and the wing planform.

A stall is when the Angle of Attack (AOA) of the wing in to the oncoming air get's beyond a certain angle, the "critical AOA" for that airfoil. Different airfoils have differnet critical AOA's, though most are actually pretty close to each other.

Wingloading matters because it measures how much lift the wing has to produce. You get more lift from a wing by increasing the AOA of the wing (assuming a constant airspeed). This means that a heavy plane flys at a higher AOA than a light one of the same design. As you slow down, you have to increase AOA to make more lift. But if you are already at a higher AOA, you don't have much room to increase the AOA before you hit the critical AOA and stall. This is why heavy planes have to land faster.

When a wing stalls it doesn't loose all lift, but the lift does fall off. Some airfoils loose more lift faster than others as AOA increases.

The airfoils on trainers tend to loose lift slowly as the AOA increases past the critical AOA. This makes the airplane mush forward, nose down, a "soft" stall. And if one wing stalls before the other, the result is only a very slight wing-down situation that is easily fixed. Espeically because the plane going nose-down quickly lowers the AOA, taking the wing out of the stall very quickly.

On the other hand, an airfoil on some kinds of acrobatic planes are designed so that the lift goes down in a hurry as you pass the critical AOA. This means that in the split second when one wing stalls before the other, the difference in lift between the two wings is very large, and you get a very fast roll.

And last, the planform, or shape of the wing, affects how the stall develops on the wing itself.

When a constant chord wing stalls, it usually starts to stall at the center, and the wing tips stall last. If one wing starts to stall before the other, and it's stalling from the root outwards, the difference in lift near the fuse creates a small amount of rolling force, maybe nothing at all noticeable. On many trainers, it's really hard to stall the wing tips, you stall the root, the lift goes down a bit, the nose drops, you pick up airspeed, and the AOA goes down. The wingtips never stall at all. Washout has the same effect, making sure the root stalls first.

On the other hand, tapered and swept wings can have either the whole wing stall at nearly the same time, or the tip can stall first, resulting in much higher rolling forces.

So, when you add these all together, you see that a trainer is the "best" of all worlds for preventing stalls.

A Cap is (nearly) the worst: high wingloading, acrobatic airfoil that drops lift in a hurry, and a tapered wing that stalls all at once rather than root-first.

If you notice, most recommended "2nd planes" have a straight wing, and a fairly thick blunt airfoil, and a wing that is between a trainer and something like a Cap (resulting in a mid-range wingloading).

Something like a Decathalon is deceptive. It looks "trainer like", with it's high wing. But the wingloading is much higher, and the airfoil is different. (there are other factors as well, like vertical tail size and placement that also make the Decathalon snap like it does)