Ekko can be flexed into 3 roles--arguably 4 (I have genuinely played with very good ekko supports). Karma can be flexed into 2 support and mid--arguably 3 if you count the korean meta playing her top lane.
SInce his release, Ekko has been held a particle kind of dominance with his ability and appeal and is the flavor of the month right now with being banned quite often. Im not a particular karma player; i used to main her support but i stopped when she was nerfed AGAIN, but i had a recent mid game on her and she's enjoyable and holds up incredibly well with zoning.
If youre looking for something that is completely versatile in nearly ever aspect with hard lategame go with ekko. Karma is a high damaging zoning bully early and midgame, that can be a solo lane utility carry. I think you should pick the one youre more in need of adding to your roster of champions available to you at the moment. You can "carry" with them both, just in their respective ways.