I looked at the Suntronic link and would like to make/repeat the point that some of these controllers work in different ways and you should be aware of what it is doing and how. The Suntronic device is a burst fire controller while the other devices we were looking at are phase angle controllers.
Whole cycle, or burst fire control, in which the thyristor is switched on at the start of a supply cycle and switched off again after one or more cycles (or half cycles).
Phase angle control, in which the thyristor is switched on (fired) at some delayed time after the start of every half cycle, and switches off at the end of the half cycle (when the current falls to zero). With zero firing delay the whole half cycle is conducted. As the firing delay is increased less and less of the half cycle is conducted.
Burst fire means the heating element is at full power for some length of time, multiples of around one second in the case of the suntronic device. The on off cycle is varied to give different average power but the power is always full when it is on. So if you set the control for half you will not get half your supply voltage powering your heater you will get all your supply voltage powering your heater every other second.
AC voltage goes up and down in a sine wave varying from zero to peak voltage. Zero is across the centre line of the sine wave and the half cycles above the centre are one polarity direction and below the centre line the other direction, hence Alternating Current. Phase angle control gives a variable voltage output by choosing when in the cycle to turn on. Phase angle control effectively clips the tip off the sine wave and so limits the power provided according to how much is clipped off. So if you set it for half you will get half your supply voltage powering your heater.
Arguably phase angle control is a better option apart from the possibility of generating radio interference. This is not always a problem in practice but can happen.
Both work though and opinion is only that. My personal preference for phase angle is just me. Others will have their own opinion.