A UBEC (Ultimate Battery Elimination Circuit) is basically a step-down voltage regulator. I feel that the jargon deserves a bit of explanation and history, so here goes:
In hobbyist grade remote control cars/planes/boats/etc. the electronics (receiver, speed controller, servos) need a power source. With engine powered craft, a small 6V battery pack was used to power the electronics. When electric motors became more popular, people wanted to use the large motor battery packs to power the low-power electronics. Typically, the electronic speed controller absorbed this function, and it became known as a Battery Elimination Circuit (BEC). With battery packs usually in the 9V-11V range, the electronics would probably need 5-6V to be happy.
Evidently there has been a push to use higher voltage battery packs (10V-25V), probably to take advantage of the brush-less motors. As a result, if the servos draw any appreciable current, a linear regulator would burn a lot of power. Obviously, when your flight/driving time is based on how efficiently you use your battery, a linear regulator is not what you want. Ultimate Battery Elimination Circuits are basically separate regulators (usually switch-mode) that deliver 5V-6V at hopefully high efficiency.
Now for the comparison. Your parts basically have two different end-use requirements. The Dimension Engineering product tries to match the form factor of a common linear regulator (7805). It would probably integrate better with any finished PCB you would make, and has a metal shell which hopefully shields EMI. The Hobbywing regulator is a more cost-conscious physical design, with a bit better efficiency spec. Honestly they're pretty much the same thing, so you could probably go with the cheaper one (Hobbywing).
I think you're a bit confused about what these parts do !
The THD 12-2412WI DC-DC converter is an isolated power converter. It is like a mini-mains adapter but with a low voltage input. It takes power from the input and using switching and a transformer makes the power available at the output. But as it is isolated, there is no electrical connection between in and output. This is useful for many applications. One field of application is sensitive measurement equipment where the sensitive input amplifier needs to have it's own isolated power supply. You could achieve the same with a battery but batteries run out which is less practical.
The TSR 1-24120 switching regulator is not isolated. Indeed it is most likely a buck converter. Note how it has only 3 pins, my guess is those are input-ground-output just like on a linear regulator. I'm also guessing that these are a drop-in replacement for a linear 3-pin regulator. As it is a switching converter, it is more efficient than an a linear regulator and it does not need a heatsink. The disadvantage is that this device will be more expensive. On eBay you can get cheap modules based on the LM2596 chip that basically do the same. But check the specs to see if it fits your needs. These more expensive TSR modules might give a "cleaner" output voltage. But it depends on your needs if that is relevant.
So both are switched converters, both are DCDC converters ! Only one is an isolated DCDC converter. Isolation requires more components and also more testing (the manufacturer wants to guarantee good isolation so they probably test it on each device, testing is expensive) making the device more expensive and larger.
Best Answer
Switching regulators and switching controllers are very similar, and essentially perform the same function. Both of them are DC to DC converters.
Both switching regulators and switching controllers can be obtained/configured in either buck (output voltage < input voltage), boost (output voltage > input voltage), or both topologies.
The buck/boost mode is useful for battery operated circuits, for example you might have a 3.3v circuit, powered from a 3.6v battery which is initially charged up to 4.2v. The battery voltage drops to 3.3v while in buck mode, and then falls further below 3.3v where it uses boost mode.
The circuits uses a combination of one or more FETs represented by the little switches in the diagrams above) and an inductor to carry out their work.
Switching regulator ICs contain all of the necessary hardware, except for the inductor and a few resistors and capacitors, inside the one chip. In particular, the switch mode FET is inside the regulator. As a result, these chips can't handle too much current, usually only an amp or two, otherwise they would get too hot. Here is a typical circuit for a 24v to 3.3v 2A buck type switching regulator:
With switching controllers, the switching function is done external to the chip. This allows for much higher currents than switching regulators, since the controllers themselves don't have to handle the current -- just the external FETs which can be sized as necessary for the task. Here is a typical circuit for a 24v to 3.3v 8A buck type switching controller:
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Switching controllers also offer a lot more configurable options, which is why this circuit is considerably more complicated than the previous one.