From my perspective power is the best way to go. The ninjas will eventually come. You can make a great team 100% f2p without even spending coupons. Darui blitz team... Mei, ten ten meta etc... These teams are 100% f2p and will take you a long way. That's why they are so common. Even the mei meta is used in china to this day. Once you have your team focus on power! More ninja will come via free events or treasure pulls. The premium ninja you c*so get over time by getting a few frags in events that dont offer a lot of power.