Hydro has way more benefits, but will also cause way more trouble.
Hydro is a bit more advanced (not hard, just needs more attention and research, starting knowledge etc.), but will yield better results overall. And it will do so with a higher chance than CO², which only works to your advantage
if everything else is perfect, otherwise the plants will not make much use of the CO².
There needs to be excessive light, there needs to be more heat and humidity and plenty of room to grow. and if you grow in a 3 gallon pot in earth, no amount of CO² will help. As a matter of fact, I would not even consider CO² before I would run it all in a hydroponic set up to start with.
But you don't have to go full out tech for the CO². Instead, get the hydro setup, and then do a DYI CO² drop. Something like this:
A small canister in which you feed sugar-water to yeast (bakers/brewers yeast), with a small air hose sealed into the lid, leading deep into a smaller transparent bottle, filled with water (this way you can see the CO² bubbles moving and know that it works), which has an air hose attached, spreading the CO² into the room. raise it high enough (CO² is heavy and drops to the floor, good air circulation helps moving it around) and you can still get some additional CO².
It does not seem like much, but two-three of these in a grow tent of your size will at least keep the CO² at a slightly elevated level.