One down side to vinyl fencing, it that the horses can just lean on it, requiring an electric wire.....therefore your vinyl fence is only as good as your electric wire!
Also, the other thing that bugs me about vinyl fencing, is that if it gets all warped from a horse leaning on it (because someone forgot to turn the power to the electric fence back on!) is its a bugger to get nice and straight again and because it's one whole long roll it's a pain to replace!
I like good solid post and rail, easy to source, easy to mend, strong! Plus, if someone forgets to turn the power on to the electric wire, no worries, the worst thing is a horse who decides he's going to have a chew on a post....they generally wll respect a good solid fence...and it's even better with a bit of electricity running around the top rails