When you discuss amps and volts, you're measuring different ways to describe the same thing behaving differently. We'll rely on the time-tested method of describing volts and amps first, then talk about what they may or may not mean regarding power tools.
The time-tested visual explanation
Think plumbing pipes and the faucet in your kitchen. The voltage, which is measured in volts, represents the water pressure when you turn on the faucet. The current of the water, or the rate of flow, represents the amperes or amps. You get the picture.
What this means for power tools
The tools are rated according to how many amps they can handle over a specified period of time. The longer an electric tool is run, the hotter the parts become and the more likely they are to fail. In essence, the amp rating tells you how effectively the tool can dissipate the heat created by moving parts working at a high rate of speed. It is counter intuitive, but the faster a motor can spin the more air it draws in to help it cool down. You might think it would just get hotter. So the amp rating is telling you how long you can run the tool without it burning out, not how much power it has.
Volts are the workhorse
Especially when comparing cordless power tools, think of its voltage as horsepower. Do you get more horsepower out of a five-liter turbo-charged Mustang than you would from a hybrid battery-charged unicycle? Of course you would. So the higher the voltage the bigger the job it can tackle. So if you plan to be cutting a lot of pieces of lumber building a house or deck or whatever, you'd want the higher voltage.
Duration versus strength
If you're a contractor that relies on his power tools for heavy-duty cutting, screwing and driving, you'd likely favor the higher voltage tools. Besides, it just seems more manly to have an 18-volt screw gun over a 14-volt screw gun. But the evaluation should include quality of the components, as well. If you can buy an 18-volt circular saw at the same price as a 14-volt circular saw, you have to ask yourself why. It could be that one is built with better, heavy-duty parts than the other. So amps and volts aren't always the determining factor.
As important as amps and volts are when considering on which you should place more emphasis, is the use. If you're a home-maintenance guy with a lot of interior or detail work to perform, 14 volts and higher amps might be plenty. If you're a heavy duty home-framing contractor with a lot of quick but big cuts to make, the opposite might be true. The bottom line is what you plan to use the power tool for most of the time. If it's hanging curtain rods, low amp and low voltage should serve you well. If it's for big indoor and outdoor projects, higher voltage and lower amperage could be the best bet. The use of the tool should be the overriding consideration.