The fact that one DOESN'T have cintrol over many variables in clinical resesrch is what makes it a not good basis for research experience, as a rule of thumb in my mind. It is more complicated and less about science than good epidemiology.
Well, now you're basically defining "research" as "basic research." Clinical research is still research if you don't have experimental control over all the variables. It just makes you think of clever ways to control for various factors and isolate the variable you want to measure. It's just as good science as basic research - just a different kind of science.
Basic and transl is what teaches you the fundamentals best because it can be controlled, and can be straightforward.
This hinges on your definition of "fundamentals." Sure, basic science research will teach you the fundamentals of basic science research well. But that's not the question. The question is, which kind of research will teach you about the fundamentals of research best? Well, what are the fundamentals? The fundamentals are coming up with a hypothesis, coming up with ways to test that hypothesis, gathering and analyzing data, and communicating your results. So basically the scientific method. This is invariable across the fields of science, whether you're talking about basic or clinical research. Both will teach you the scientific method. These key tenets of the scientific method are what is important to become a good researcher.