do the math. if you look for an app called light meter and use it at the canopy and see what light is there and draw a line around the area with enough light to flower (google how much you need, but like 60k) make sure you are measuring where the canopy would be at flower. then imagine the plant big and full and will it fit? i say two will fit. so to me, it is not a choice thing, it is a fit thing. the 5 gallon lets one plant grow a bit bigger and use your space.
you can find threads here on this.