I think I would just use a regular rope. Every broken halter or lead rope that I have ever seen failed at the hardware; a broken halter ring or trigger snap, not a snapped rope. I've never tied a horse by a chain, but it stands to reason that more metal components = more places for it to snap. It depends on how heavy the chain is, I suppose, but I've seen beasty thick trigger snaps broken clean off by a panicking horse.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown