leather or nylon surcingle?

Depends what you plan to use it for really, low level/starting the occasional horse nylon is probably good enough. If you’re planning to do advanced in hand work/multiple horses then it’s probably worth investing in a leather one.
 
Advanced, like if you’re aiming for Spanish riding school type work, advanced lateral work and collection. If you’re just trying to get a horse to learn to relax lunging then that’s just very basic work.
 
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