Really no benefits, it's more person dependent and preference on what works best for them. I think ring shape and type like a myhardwear, or material used Should be a factor as well for optimal results. That is also person dependent.
I'd say most people that have done both swear that it works better behind the balls in keep blood trapped better. I agree that it works better and can keep it fat pump all day behind the balls. Most people that use an all day
cock ring, do this as well have said it's given best gains. If I use just shaft I loose it in a couple hours. Behind the balls I can keep a very flush flaccid all day. Others don't like it and complain of pain, feeling weird. I feel for me it at least during sex it delays ejaculation much more efficiently. And if it'd a hardwear adjust type the hardest erections of my life.
For say after pumping and sex Shaft wise a silicone works fine during sex and most pleasant for most partners, cause steel is cold and hard. But I find silicone aren't as good as proper fitting steel ring for retaining post Girth work out as a proper fitted steel. For me rubber silicone cause irritation and that leads to bad turtling and rubs me raw.