should I use co2?

or save the money for an hps?

Save $ for both. HPS first.

MH seems to work though

OH yeah MH is great, it works for sure. Im using 600w MH right now for veg and flower. So I guess now its your temps that should decide which to buy. If your temps are good then get the HPS. If they are high then go co2 to compensate

they average high of 80 at canopy
low of 72

Then your temps are good. I havent had the chance to use my 600w HPS to flower yet so Im not sure what a difference in yeild would be compared to the MH. But id still go with the HPS then the co2