The clothing is also an option. It will cost 3500-4000 coupon and should give 800-1000 power. And that's not counting the power that you might gain from the board itself(magatama etc) So the actual comparison is can 40 sets of charms(what you can get for 1400 pts) give you 800-1000 power.

You can get a better ratio via charm or refine if your charm or refine is still on the low to medium range. But if they are already high somehow, you could consider the cloth.

Diminishing return requires you to diversify investment.