There are no dumb questions, only dumb questioners -- those who ask and refuse to hear the answer.
Any power supply at this price-point is going to have some compromises, and voltage spikes are certainly one of them. How severe that is depends on how you're using it, of course. Adding filtering on the output can help mitigate that.
One real advantage the Tenma will have in your case is that it will just plug into your wall and work. The units this thread is discussing require a power supply of their own to work--and which one you use will have a large impact on what the supply can do. If, for example, you use a wall supply that puts out 5 V and 1 A, you are limited to (roughly) 5 V and 1 A. You won't get 20 V 3 A no matter what you do, unless you have a power supply for your power supply that can handle it.
So be aware of that; these little modules are slick, and seem to be pretty well made. But to use them safely, you'll also need a good wall supply (which will cost as much as the module), and a decent case (which if you buy from RD Tech will again cost about as much as the module).
An equivalent to the Tenma you've listed using these modules would require:
RD DPS3003 (18.03 GBP)
RD DPS Supply Housing (17.60 GBP)
Wall Power Supply something like this (26.80 GBP)
With those choices, you're looking at roughly the same price between your two options (which really shouldn't come as a surprise). For a beginner, if the choice is between these two I would recommend the Tenma--fewer parts, and ultimately cheaper in the long run. The real advantage to these little modules is how easily they can be incorporated into other projects--they're compact and work pretty well.
That, of course, can be completely off if your goal is to learn to build things yourself, in which case a housing and a reasonable ac/dc converter can be made for quite a bit less (though likely not enough less to make this the cheaper option by much). Building a power supply is often considered a really good beginning project, and there are lots of resources out there. In this case, it would be simplified by not having to make it a variable supply--it only needs to be designed for >30 V fixed with enough capacity to handle more than 3 A of current. Then you would use the RD DPS module to provide your variable voltage and current outputs. Not a bad little project! Just be aware that if minimizing expense is critical, using a module like these will invariably incur expenses for other parts and could easily make it the more expensive route in the end.