I think it depends on how much you shoot. You can get a tank for cheaper that is used and still has a few years service life left. There is a 15 year service life, and a 5 year hydro requirement. Both must be in periodicity or the Dive Shop won't fill them. In San Diego its about $5 per fill to 4500 psi. Once the tank is greater than 15 years old it can't be filled anymore - legally. If you have your own compressor then it doesn't matter. These tanks are all designed for a 30 year service life, but DOT restricts it to 15 years for political reasons. The 45 minute (66 cubic feet) tank will fill your AA many times, probably more than 15. A larger 60 minute tank, or 88 cubic feet, will fill it probably 20 times. Keep an eye on eBay for a 60 minute tank that has at least two or three years service life left. They are usually around $110 to $140, in that range.
If you shoot a lot, and have many guns, I'd look at getting a Compressor, but you'll still want to use a tank, since filling the gun from the compressor will turn into a pain very quickly...