You should go with the 5 gallons, and I would personally give serious consideration to putting up a scrog screen. You can really maximize you yield by evening out the canopy. That being said if you were to do so, in a 3x3 space you may only need 2 plants, though with only a month of veg it would really depend on the genetics to do it right.
Oh ya why not veg under the 600w if you wanna grow some bushes!