As a F2p player, i think it is important to get close in power to any mini-spenders (definitely not Kozo or Memory of the past). Like many others have suggested, staying close to power with small spenders (being top 10 in the server) is important. If you're weak, it's easier for you to get plundered in convoy, lose in matsuri/SWB (which isn't really relevant anymore), can't do ninja exams because of lack of power, can't do TI, many many more. It's really rewarding when i, a F2P who focus on power can beat a semi-P2W guy with 2k power above me (but less initiative than me because i was stacking medium refines) :D Right now For me, my 1st target is to get to 70k power so that i can get the clothing for free which gives me more power. Then i'll worry about other important ninjas.

As far as coupons go for events, i'll go for ninjas that are certainly gonna "break the meta" like Sailor Sakura, Death Possessing Hidan (was still kinda relevant) AND that you cannot get it from treasure pulls. So no thanks to Bubble boy, Rasa, Gengetsu etc... I can get those for free later. Most often what i do is compare the ninja frag prices with the power (like medium refine) prices. Each ninja has a price in my mind. Like I'll rate Sailor Sakura above masked obito even though masked obito costs way more per frag. If i can get way more refine runes (in terms of total coupons worth) more than a special ninja (like 30 medium refine or 2 frags of minato) i'll defintely go for power. Rarely would i go for ninjas unless 1) there's no good item for power 2) That ninja is SO CHEAP (last week i got like 30 sailor sakura frags for free)

I like Hanzo, even though he's not good in a normal team, he's SO SO SO useful in RANKED BATTLE. So i'll try to get him in the tree event.

So to answer your question: Focus on power unless that ninja is REALLY cheap/ it's your favourite ninja of all time or something :V