In terms of coupons, 6x card gives you 10800 cp, while jonin medal 3600. Thats 7200cp difference.

Now look at all the utility jonin medal provides you (alongside with 30 naruto frags, but thats only for higher spenders, since ftp/low and even medium wont ever get him to 4* to make him really good). Imo this utility is really helpful and lets you enjoy the game more, you have less restrictions with sweeping, how often you need to come online and when (since u complete quests with 1 click). There's also small power boost (not that much tho), decide yourself. After playing for a 5 months with medal, i dont really want to play without it again.

Edit: Zelgadis~, right. missed that. Updated the post, thx for correction :)