Iruka is good indeed but is a free ninja (his fragments are the rewards from spending 3k points you got by refining ninja fragments), so the real question is if he is worth the investment of 3k coupons when you have guy for free that do most of what he does.

The answer i would give you if you ask for a tip is: bring sasuke/naruto/sakura to 5 stars and begin to sell their fragments to get points for getting iruka frags for free in ninja refinement, in the meanwhile go on with guy and spend those 3k coupons in training potions. Getting 2500 more power in early servers do way more than getting iruka 2 months earlier imho.