This article goes into a lot of detail about them and has sound samples for the different types. There is a surprising amount of variety in resonator guitars and not just material. There are samples for those differences as well.
In addition to variations in material there are various resonator designs
Biscuit-Bridge Single-cone Design
The simplest of the designs is the biscuit-bridge single-cone. The biscuit-bridge cone looks basically like an inverted speaker cone. The biscuit bridge gets its name from the little wood disc (generally made with some solid hardwood) in the center of the cone.
The metal-body biscuit-bridge resonators were highly favored by delta blues players because of their loud metallic tone. When used in a wood body, the tone has less crispness and a bit less brittle, but still has a strong fundamental tone.
The Tri-cone has, as its name implies, three cones and a T-shaped bridge . These instruments are more difficult to build, so they tend to cost more than a single-cone model. The saddle of the Tri-cone sits in a slot along the long leg of the T. The sound of the vibrating strings get distributed to the three cones and the vibration of these cones combine/interact within the body of the resonator to create what can be described as a more distributed complex sound.
Spider-bridge Single-cone Design
The single-cone spider-bridge design gets its name from the spider-like look of the bridge. The cone itself has a W-shape to it and the spider bridge contacts the cone in both the center and along the edges.
In addition, often these cones sit in special sound wells that modifies the tonal character even further. The best term I can think of to describe it is a more "nasal" tone.
EDIT: The site contains actual sound samples. All info is quoted is from "Resonators Explained
... those "other" guitars
by Paul Kucharski
September, 2004" incase the site goes down in the future.