Using R6 can be a good choice for beginners as there are some benefits I can think of. For example, Animating will be much easier and will take less time to make. R15 will give you a more realistic feeling and requires alot of animating if you want to make realistic movements. It might take some time to finish, so this is the best benefit I can think of. I personally see many people using R6 in games that I play. But it’s really up to you, the creator, on what you want to choose.
It depends on what type of game your making. If your making a roleplaying game, it’s best to use r15 because people who play those types of games typically like it more. If your making an obby, especially a hard one, it’s best to use r6 because the extra parts could make going through killbricks more painful.
It all really comes down to your preferences for your game and it’s art style. I prefer the style of the R6 rig, but I enjoy how wide the animation possibilities of R15 is.
R6 is good if you don’t want players to use emotes (since emotes cannot be used on R6 avatars usually), and on the other hand, R15 is great for emotes. R6 is good for classic Roblox games (like what Liam said), but don’t support as many animations.