For me, it was important to source the school closest to the teachings of the Buddha. The scholarship in the area of the etiology of the Dhamma reflects that the Pali Canon captures to a reasonable degree of scientific certainty the teachings of Gautama Buddha. Many of the Ajahns and teachers that teach from the Theravada and Early Buddhism perspective are highly credible. We have today teachers like Vens. Thanissaro, Bodhi, Brahm, Gunuratana, and others who are highly intelligent people (some with strong science and research backgrounds) who likely wouldn't waste their time on a fool's errand. Lifetimes have been spent on this Dhamma, and have yielded sound roadmaps for the navigation of mind/ life and release from samsara as the Buddha intended.
After starting with Korean Zen, and spending some back and forth time with other Mahayana traditions, when the time came for me to commit to a practice and a school, there was no question other than choosing Pali Canon/Theravada. There is so much positive to be said of Mahayana, but to be critical, in some respects Mahayana has taken the Buddhavacana and created a practice out of completely new cloth. The Buddha's Vinaya is rejected. The Canon is displaced by 8th century fabrications that were geared more to nationalistic concerns, than Dhamma. Buddha is said to have made statements in later sutras that no independent scholar accepts as valid or true.
There is so much cohesiveness, intelligence, wisdom and authenticity in the Pali Canon based schools, that to practice otherwise would suggest a rejection of Buddhism in favor of, for example, "Dogenism." Try going to a Zen sangha and learning jhana. It was the Buddha who advised his monks to practice jhana, to meditate in a certain way, and this practice was later rejected by Mahayana schools seeking to "brand" themselves in a more populist manner. All forms of meditation are beneficial, but it seems to me important to practice meditation the way that the Buddha taught it.
It's a bit like the barrel analogy. There is a beauty and simplicity to a well made oak barrel. Start creating cracks and pounding pegs into it, and soon it is no longer a barrel, and it no longer holds water. Maybe I'm a jerk for saying this, but the Dhamma can be considered medicine for a deluded society, so why not try to get the antidote as effective and pure as we possibly can?