While most of the answer to this question is subjective, I'd answer analoguous to @shootforthemoon, except that it's conceptually nigh impossibly to start learning both literally at the same time,
When you say:
"During my initial research, I have encountered some people saying ..."
It does not appear as if you had done any research on the language itself. Learning the history and linguistic facts can work well in tanndem, and the history can be hardly separated at all. I'm sure you have however common knowledge, not the least because, as the prior answers hinted at, similarities to Grecko-Roman culture can be found in many layers of our own, including language.
An intensive introduction is of course more efficient if focused on one coherent language. Which one that should be depends on the value that you expect, which, yes, depends on the difficulty, but an introduction is easy, pretty much by definition. I mean, you don't expect to become proficient immediately. It should get you far enough to recognize enough differences; it should establish coherence.
After that, you never stop learning so you eventually will learn both in tandem, unless you learn just one of them.
You might as well ask on which one you should focus. But that depends on your use case, what you want to read, or whom you want to write.
Since other have already answerd in favour of Latin, I will try to argue for Greek, or generally the less favoured language: It might make sense to start with the less interesting one, to save the best for last, as they say. That might as well be Latin.
There's a bit of a circular argument, when those proficient in Latin but not so much in Greek described Latin as easier, if that's due to the longer Latin tradition in education. I mean, yeah, inflection patterns might be coherent in Latin, but that's only relevant because Latin has so much of it. I'm not sure that's exactly easier. Also, many loanwords changed drastically, so that's not an argument, and it might even be a detrimental source of confusion.
Nevertheless, I can't say they were quite wrong, it may be useful to broaden ones horizont incrementally, if you try to approach any second language as a rather weird dialect of your native tongue.
Starting learn both at once, say with a trilingual dictionary, a few common grammatical patterns and cognate sets and what else, that would be a project of its own. I don't think that's commonly done. It usually works the other way around, learning both languages to reconstruct the common ancestor, because too little is know of the history to use as a basis for language education.