I bought an aluminum tank with my original setup. It looked nice, but the only way to keep the tank was to take it to a fire extinguisher company. That used to be $15 to $20 to get it refilled. Air Gas (welding supply company) will exchange empty cylindars with filled ones for about $10. They are a little beaten up and steel... but I actually like the extra weight. (The lighter aluminum one would tip over much easier and can ruin the regulator)
Later, after playing with the 5# tank, I upgraded it to a 10# tank. It lasts WAY longer, and is about the same cost to refill. (And it is heavier than the 5#, so it is very stable)
I have gone the rout of using AIH for kegs over the past few years, I have 8 5 gallon used kegs that I like a lot, and recently bought 10 3 gallon used kegs from them. All were great, just needed some cleaning and they were ready to go.
I really like the 3 gallon compared to the 5 gallons... Lot easier to move around, and I recover space in my fridge twice as fast. (A half gone 5 gallon keg...still takes up 5 gallons of space....