You can get sage naruto in the sage treasure so you don't have to spend coupons. Hokage minato rarely shows up in events but is most of the time can be purchased via ignots, which means you can't piece him with coupons. He does appear in RNG events but he isn't guaranteed after using coupons/ignots. Jonin minato can be pieced over time as he appears in several events where you can buy or get free frags over time. If you think you are lucky, go for hokage minato but if you want a guarantee, then jonin minato.