This is the hardest thing for a free to play player. My thing is, I set goals for myself. What I want to achieve and that is what I focus spending my coupons on.

So if you've been wanting Iruka for a long time, choose him. If you're not sure if you really want him, don't get him. Seal scrolls aren't worth the investment, for that I would wait for a good event if you really wanted seal scrolls.