With very few exceptions (Li, Boldtusk, e.g.) levelling a 5* is better than levelling any lower ranked hero.
Below is viv-enn’s team, who I have fought maybe half a dozen times now and I win on attack in almost every match-up because Kadilen’s special buffs for the turn it’s cast and then the next one. Essentially I only have to wait one turn to trigger my own specials which is not long enough for viv-enn’s team to counter anything that may be coming.
So, as it stands for your green slot, Caedmon is a better choice for levelling, if you have him. Otherwise level Kadilen and hope SG buffs her special to 3 turns or more which I think would make her as effective as she should be. Kadilen should be one of the most powerful 5*, instead she is one of the weakest because her special is broken imo. Her special could probably be buffed to 4 turns without overpowering her, definitely to 3. Devs please note.