What strains are you growing? You might only be growing two plants; but those might be the stinkiest two plants known to mankind. The strain will greatly affect the smells produced by the plant.
What are the temperatures inside the tent? Terpenes and terpenoids become highly volatile at higher temperatures and are constantly being created and destroyed during growth; if you have fluctuations of temperatures you may release more or less odors into the air.
I use ONA products; I personally think ONA is a great product but there are some things to note about essential oils products (which ONA is) versus a carbon scrubber. The ONA releases essential oil molecules into the air which binds to the odor causing molecules of the plants, forming a new odor compound (that doesn't smell like cannabis.) It doesn't REMOVE the odor causing molecules, it just attempts to bind with them to form a new compound. Here is my suggestion if you use ONA concentrate or gel, keep a low speed fan on it to help diffuse the product into the tent; it really needs a way to be distributed properly into a space (unless it's a small space, then it may do just fine on it's own; it works well in my small tent (3 ft by 2 ft by 5 ft.) The problem with essential oil products is that there are many essential oils, each with different capabilities to bind with certain odor causing molecules, and there are many terpenes that form on cannabis. So depending on what you're growing, ONA might work really great, or it might not working very well. So just understand those limitations.
The carbon scrubber actually traps odor causing molecules in tiny micro pores in the carbon. So instead of trying to combine with the odor causing molecules like the essentials oils, it just grabs them and holds on to them. The activated carbon is crushed charcoal; the more surface area exposed; the more exposure to trapping odor molecules.
The scrubbers, in my opinion, are hands down the best option if you plan on doing this at all long term. I think you would spend more on ONA through the life span of the scrubbers and the scrubbers are by far more effective when properly paired with a fan. ONA most certainly
could get you by, I actually have some ONA mist, gel, AND concentrate in my smell kit; but I also run scrubbers. I'm just paranoid. Really. Really. Paranoid. But that's just my