What's the difference between the hard and soft stoppers and which do you prefer?
The difference isn't huge but if I was given a choice, it'd be the soft. If I wasn't given the option, then it's far from a deal breaker.
I assume that they simply slide over the spar, no zip-tie, etc? It seems that the idea is that they remain 'mobile'. I am also assuming that I will need to remove the entire spare from the kite in order to install the new stopper.
And here you come across the problem with these little buggers:-
It can be a major rebuild with spars, fittings and bridle lines all over the shop just for a small bit of plastic. And so ingenuity comes to the rescue !! What you do is cut a slice out of the opposite-side-to-the-horn and drill a hole at the very base of the horn. What you end up with is an elongated c-clip with a growth. You can then slip them on without the wholesale reconstruction and use a tie wrap to stop them from rotating on the spar (which is to be discouraged as they have a habit of hiding).
I'm sure I can find a picture if this all sounds pure gibberish.
Another option is the horrendously clunky but functional Kitehouse blobs that do the same thing as above.