Hi! There is no best method really, it comes down to personal preference. LightMix is really originally intended to be for when you know you will want to adjust the lighting after rendering, e.g. you know your client wants an evening, noon, and early morning shot, or they want to see the same shot lit by colder blue LEDs version warmer yellow conventional bulbs. In which case, there is no choice other than LightMix (well, except for rendering twice, but that's time consuming :) ).
However, now that LightMix is there, a lot of people have adapted their workflow so that it is "don't care so much about tweaking the lighting before you render, do most of that after rendering." Which is perfectly fine, and just a matter of choice usually (and then it opens up the option to respond to client requests of "the sun is too bright" without having to re-render... so kind of an "insurance policy" in that too :) ).
The only exceptions to it being only a workflow choice is a) LightMix uses more memory so that might be a concern if your machine has limited RAM and b) LightMix may take slightly longer to render due to having to denoise all the LightSelects (still faster than re-rendering the whole scene multiple times, of course!)
So overall, in most cases, either approach is fine, whatever the artist prefers :)
