Aluminum rings are very lightweight and non-bulky. Nylon rings are also lightweight, but they are a bit bulkier (the actual ring is thicker than the aluminum rings). However, nylon rings will not become hot in the sun while aluminum may absorb some heat from direct sunlight. We find aluminum rings to be more aesthetically pleasing than nylon rings. Due to the larger inner diameter, aluminum rings are also slightly easier to adjust, although nylon rings tend to "grip" the fabric better. You'll probably be happy with either choice, but we want to give our customers the option as it's really more of a personal preference, than an issue of safety, adjustability or comfort.