Without CO2 in that tank you will not get plant growth. You have tons of light, dosing ferts, etc. I would get whatever size tank will fit better. You don't want to regret getting the 5# tank just to save time on replenishing the CO2.
Like epic said, go with GW ferts. Read about dosing ferts as well. Pouring ferts into a tank without knowing how much to dose, plant uptake, etc. will be no good. Ferts are extremely important but the amount to dose is totally different per same size tanks. Its all about plant uptake IMO.