Green or red?

Not a Lupin the third topic, sorry.

Seriously, if you want to spend your few gems (the monthly ones) and you already have Boldtusk and Caedmon, which element you choose to do a call?
In other words, what 5* i should aim for to get a better team, taking the spot of the mentioned hero? And why?

Generally speaking, i know i have to consider the synergy with the team.
Let’s just say it’s not a defensive pack.