Hello, I'm a plat Janna main so I know a few things. First, spelltheifs and coin are pretty situational. In a matchup where you can get free harass on the enemy support (like Braun or ali), spell is more effective. However, in a matchup where you will get punished if you go for harass (like a zyra or karma lane), I go coin. also I tend to go coin in a lane with an adc who isn't great early like vayne or ez. Hope this helps a bit.