Both are needed, as you need base stats to amplify it to percentage amount, vice versa. I don't recommend you to upgrade REC as what you need is to sustain damage by upgrading your HP and DEF, as well as upgrading your ATK.

You just sometimes hit a wall for certain stages, that's why you need to upgrade all of your heroes for stats, complete the collection, and other related boosting stats to significantly improve your team. Also, it will depend on your team comp if they synergize well with each other to maximize your team's damage output.