I’ve always argued Infernus before P-R but it’s really personal preference.
Getting Infernus first is a major boost to your abilities in every game mode, but how much of a boost depends on what kinds of teams you can build.
Getting P-R is a major boost to your soul farming ability, and you’re really likely to be able to build a decent soul farming team.
But you’re only going to soul farm for about ~1.3 worth of souls. Once you do that, you’ll be at a point of the game where you get enough souls “just playing” to make pretty good progress without farming. So eventually you stop using Pharos-Ra unless you really want a slow-but-strong sniper. I haven’t used it in probably 5 months.
On the other hand, Infernus is always going to be among the “strong” mythics. You’ll keep using it over and over again. I used it this week.
I did P-R first and didn’t like that path, but I don’t think people who say to do it are wrong. I just think I’d have enjoyed Infernus more. (But also keep in mind there are several new, non-Infernus strong choices that have released since then. When I did P-R, Infernus was unquestionably the strongest troop in the game.)