It depends on what you plan on working with. What would you like to use them for?
For example, if you plan on working on a Wii, 15v and 3a should be plenty. That'll give you enough voltage to power the stock regulators, or enough amperage run any individual voltage line.
My PSUs are all 30v 5A, but I can't recall if I have ever used more than 15v or 3a for reasons other than stress testing.