Bettas are known to live in small confined areas. However, the larger the tank, the better. The minimum space to own a Betta is 5 gallons. I agree with mattgirl that you should go for the 10 gallon tank
The Betta may not withstand the large currents of a 30 gallon (unless your curent is small)
Ok 10 gallon it is! I have another betta in a nano 40 gallon long and he seems to absolutely love the space but was thinking 21 inches might be a bit to high. Win/win an empty 30 cube..oh the possibilities!!!