Wait until the events are available to determine. It's hard to tell what is worth to spend on. Also, do you already have minato? It would be less work to use your coupons on him.

If you happen to see that both minato and kimi halloween is available, I would save it is worth to spend the full amount. If they are available focus on minato until you get him to 4star and use all the remaining coupons you have on kimi.