That’s a very interesting question, in my experience it varies a lot from person to person but there’s a constant to which I have seen no exception: the more languages you already know (as in, you can read and speak fluently), the easier it is to learn another one. If a part from your mother tongue you already know another language or two I’d say it’s worth a shot to try both, if not I’d suggest to focus on one (specially if you’re going to spend money on it).
I only saw the first five or so lessons and I can tell that it is very clear in grammatical explanations, and that’s precisely the reason why I disliked it, it focuses too much on grammar rather than comprehension.
If you care to read about the most recent discussions in this forum (like here or here) about methodology you’ll probably notice that my position on language acquisition can be summarised by your statement. I deeply believe that it’s better for the student to intuitively pick up the grammar, to induce the rules by himself and only later to see the arid grammatical explanation.
Anyway if you’re interested in what I consider to be the very best Greek Course available with lots of minute and detailed explanations, I suggest Zuntz’s Greek Course (my review here).
Or if you want to take a look at another type of pedagogy why don’t you try Athenaze (there’s also the original but not so good English version) or Reading Greek?