PUBG runs on the Unreal engine, Free Fire is better suited for low-end devices. When it comes to graphics, PUBG Mobile stands out. PUBG Mobile uses Unreal Engine, which is very popular among PC gamers and this engine delivers very smooth and realistic graphics.
In short, if you have old hardware then play FreeFire.
If you want all the gaming juice in the world and you have hardware to support it then play PUBG.