Chain D-ring loop: Ultra-strong, ultra durable, more suited for bully breeds. OR for greyhound owners who know to only use a martingale for walks, and know how to adjust. Also, a 1" chain loop will cinch up must faster than a 2" fabric collar. Think about it, more surface area to go through the tri-glide.
Fabric D-ring loop: More suited for dainty dogs, like whippets and greyhounds.
All in all, I prefer the fabric loops for sight hounds and chain loop martingales for bully breeds.
A note: When doing your shopping, PLEASE look at how long/amount of sales vs reviews the shop has. There are some very frightening new collar shops on etsy, with extremely inadequate thread and stitching technique.