Thanks for the opportunity to talk a bit about ring mandrels! Although the plastic one you mention is called a 'mandrel', it really is a ring 'stick' which is great for finding out the size of a ring that someone wants to have measured, or for making certain designs such as smaller style rings or some seed beaded rings, but not very stable for beating metal or wire rings.
A wooden ring mandrel is a tool used mainly by the artist who does lost wax designs, PMC, clay, etc, but can also be used in the above mentioned scenarios. A metal ring mandrel made of copper, aluminum or stainless steel is best when working in our medium of metal wires, (and over time, those made of copper or aluminum can become dented and scratched).
There are also several styles of ring mandrels to choose from. A graduated mandrel, (stainless steel), is my personal choice for making wire rings. The graduation allows a wire artist to push or slide the ring form down as a pattern may call for, to help with sizing the shank, whereas a 'stepped' mandrel will not, (it resembles the stepped side of 3-step round pliers). A grooved ring mandrel is nice for the person who makes a lot of bead rings, (and sometimes I like it when making cut stone rings to help protect the culet of a stone). There is also a ring mandrel with one flat side, for the person who frequently makes Pharaoh's cabochon rings. In my opinion a wire artist only needs one graduated, stainless steel ring mandrel.
To summarize, although a plastic ring stick is less expensive than the other options, as with any tool choice, think about what you want to make/do with the tool before you purchase it. Rather than having to replace a less expensive tool every few years, sometimes it's better to spend a little bit more on a good product that will last a lifetime!
Answer contributed by Dale "Cougar" Armstrong