Buckle for me.
I have an ear shy horse and if someone tried to do the clip option with him it wouldn't go well. I have been tempted to remove the clip option from my headcollars (and replace with a welded loop) that have them to ensure it doesn't happen but I haven't got around to it yet...
I would be more confident if the horse was already doing the job you want it for, eg has been ridden by a teen who moved up from a pony but they are now off to Uni. My reservation is that I have known ex-racers that were never safe for say a group gallop on a stubble field with a rider that did...
How long is it since the horse retired from racing?
Have they been re-schooled?
What job are they doing currently?
How are they out of the school eg in open spaces?
Ahhhh... the joy of fine skinned horses. They knock themselves playing about and the leg fills, they have too much protein in their diet and their legs fill and so on...
Check carefully that there isn't a tiny cut hiding somewhere on the leg, and if not just keep a close eye on it. For example...