Probably R15 is the most popular now. It just looks better than R6. I think R6 is for the nostalgic people of roblox, but it still has some functional benifits over R15, but not a whole lot. The only game I forced the R6 in is an obby of mine, because of the way the arms and legs move. Turning idle animations off and use R6 would make it possible to create more tighter spaces in my obstacles to make it more intense to go trough them like how the old days worked before. Also nostalgic games would look much better with R6 for the vibes.
So I think R15 and R6 probably have some functional use differences, but also for looks in my opinion.
This means it depends on the game your trying to make. Supporting both R6 and R15 is the most ideal, but you could choose one or the other depending on the usecase.
As a (kinda) old player, I prefer R6 because it looks better in my opinion, but if you’re trying to appeal to the newer playerbase, I suggest making things in R15. I guess it’s more fitting in today’s Roblox standards.