Since this is a general forum post- we need to address the reason for cock rings and the different styles.
Cock rings are 1. Sexy looking. 2. Some are sold to make the cock harder (not all!) 3. Some (especially Jelly) are designed to aid in the woman's pleasure. 4. Then we have the vibrating cock rings with from what I've read and reviewed are a full range of hit and miss...
The solid metal rings are not a good choice for beginners. The basic rubber rings can be, but have a lot less excitement power as well. Many cock rings are actually designed to go over both cock and balls.
Before you "Know" what cock ring to buy- you need to understand the purpose of the cock ring, and be honest with yourself as to exactly why you want it. For example- in general the jelly rings don't do much for erection support. They are fun and toys. The leather strap on ones do, but they are harder to use and get used to.