Amps vs. Volts
How many times have your heard that it's not the volts that'll kill you, but the amps? While mostly true (it takes only 100mA to stop your heart), amps and volts are eternally bound together by Ohm's Law:
V = IR
In plain English: volts (V) equals current (I) times resistance (R). So, assuming resistance is constant, voltage and current are in lockstep. As one goes up, so does the other, and vice versa. You can't have a lot of amps without a lot of volts.
So, back to which kills you, the amps or volts. Given your body is a constant resistance, it really is a combination of both. Higher voltage means higher amperage, and thus higher voltage has more potential to kill. It takes only 100mA to stop your heart. I believe the resistance of a person with dry skin is somewhere around 100,000 ohms. This means, even touching 120V is far from lethal. The probem is wet skin is around 1,000 ohms, and the inside of your body is even lower, thus making 120V potentially lethal. OSHA actually has a good page describing this (even if they resize your damn browser window). And here's another explanation of the biological effects of electric shock.
You also, of course, have to take the power supply into account. All power supplies, especially batteries, are far from ideal. As they supply current, the voltage drops. A standard 1.5V AA battery cannot source 1A for any sustained period of time (if at all), for example. That's what makes a 12V car battery a lot more powerful, than, say, 8 AA batteries. I sincerely doubt you could start your car with a pack of Duracells.
BTW, even a standard 9V battery can kill you. Don't believe me? It's happened in the Navy. And I've seen that story before, so I think it is actually true.
I also think that many people are injured or killed, not necessarily from direct contact with an electric shock, but the aftershock. For example, a television tube usually runs at somewhere around 20,000 volts. But, it doesn't have a lot of current capability. The voltage drops off quickly as it discharges (i.e. you touch it). So, even though the shock is not lethal, the involuntary muscle contractions can cause you to touch something else more dangerous. Perhaps something inside the TV (like the power supply), or maybe it'll through you off your chair and into something sharp in your workspace. Either way, the potential for injury is still significant.
Anyhow, that ends my rant on weather amps or volts will kill you. The truth of the matter is they are both dangerous in the right quantities and situation. So be careful around electricity!