Appearance of back
In terms of the appearance of the backing, the Sewing Basket brand eyelets that come in sets of 2 with eyelets and snaps are the best. They are thicker in the end, but there are no scratchy edges - round in the front and round in the back. The one piece Sewing Basket eyelets also work nicely with backing edges that curl neatly in a circle. The worst is the Making Memories one-piece eyelets. They basically look smashed.
Appearance of front
The one piece Sewing Basket brand eyelets have the most narrow edges. The two piece Sewing Basket eyelets have round, slightly thicker edges. The Making Memories brand eyelets are like little donuts, and have the most irregular of shapes after setting. I think they are made from a softer metal than the Sewing Basket brand so they tend to change shape after being pressed with the setter.
The Making Memories brand is the flattest of them all (since the backing smashes down flat) and come in many different colours and shapes, so they would be good for scrapbooking since you'll never see the back. The next flattest is the one-piece Sewing Basket brand, and the least flat (but still acceptably flat) is the 2 piece Sewing Basket brand.