I think it goes without saying a smaller system is less stable, due to the small quantity of water involved. It takes a whole lot more ammonia to cause a rise in 5000 gallons than it does, in say 50, having said that. I think the smaller system will make you learn AP faster, in that you will not have as much room for mistakes, so you will either learn to keep things balanced and monitor your system correctly, or you will have a dead system. With a bigger system a person can probably get by a lot longer making mistakes, before it really shows up or causes a problem.