Thinking of buying a ring sling - does it matter if the ring is aluminum or plastic?

It's the combination of the fabric and the ring that determines how easy it is to adjust actually. I have a plastic ring RS that is easy to adjust as the fabric is thin. On the other hand, I have used a big metal ring RS that wasn't easy to adjust as the fabric was very thick. Some thin fabrics won't work well with a large metal ring as it would be too slippery and not offer enough grip. Metal rings can come in plenty of colours. Both nylon and metal rings can be seamless and weight tested.