One reason AC is more deadly is that any path which cause the current to pass through the body and cross the heart, e.g. left-hand-to-right-hand or hand-to-foot will cause the heart to attempt to synchronize its beat to 60 Hz. The heart goes into fibrillation, and unless someone gets an AED on you within a couple minutes, that's the end. In addition, the alternating current locks the muscles in a spasm, so you can't pull away. With DC, your greatest danger is burns. The reason DC feels much worse is that it causes the muscles to contract abruptly (whereas AC causes them to lock), so the physical effect is more painful. Edison favored DC, and Westinghouse favored AC. Edison wanted to introduce the word "Westinghoused" as a synonym for "electrocuted".
A higher voltage breaks down a poor insulator (e.g., the thin layer of nonconductive dry skin that covers the body), and once that insulator breaks down, the inner layers of the skin, and the muscles, are highly conductive.
15 mA is the lethal dose. That is why GFIs are set to trigger at a current of 5 mA differential.
I have not tried the experiment, but I have read that a 9 V battery connected to two sharp needles will, if the needles are stuck into the skin, be very painful.
I have had several EMG tests, which measure neural delays. For example, they are very good at telling the difference between neuropathy of the hand (normal neural transmission from the area of the elbow and the fingertip) and Carpal Tunnel Syndrome (significant neural delays). This is done by putting a wire on one finger, and hitting me with a cattle prod. My arm jumps, the experience is painful (I once asked the technician if Amnesty International knew about him; sometimes toward the end I will tell the tech that if I knew any secrets, I'd be telling him). Each pulse is at a higher voltage; he hits me with the cattle prod, takes it away, clicks a knob, and repeats. I looked at the calibration once; the knob was set to 800 V after the last test.
In one of the more surreal experiences, and this is over 50 years ago, I was helping the electrician at the company I worked at. He used a wooden ladder, always. He was up amongst the many panels; we had 120, 240, 440, and 880 volts in that array. So he calls down to me to get his voltmeter, which is down the hall. I come back with it, and he says, "Never mind, this is the 440 line". After he came down, he explained that he just bridged two of the phases with his fingers. "It was too strong to be 220, and too weak to be 880". This was a guy who had a perfect way to locate a short. Remember, this is 50 years ago, and you couldn't buy a TDR at Wal-Mart. He'd disconnect everything from the circuit, then run a cable from the 1600 V line down to the wire whose circuit was shorted (the end disconnected from the 120 V or 240 V panel). WHAM! Wherever the short was, there was an explosion. 1600 V at about 800 A, if I recall correctly.
I have been hit by 120 VAC, and a variety of DC voltages from 90 to 20,000 V. Even the low DC voltages (remember when electronics had these weird hot glass bottles plugged in? One of the voltages, called B+, ran from about 200 VDC to 800 VDC). I quickly learned the trick of shorting the power supply condensers (now known as capacitors) because that voltage stuck around for a very long time after the device was turned off and unplugged. The DC hits were memorably painful. The AC hits were far more dangerous.
A gun is always loaded, so the rule of "never point a loaded gun at something you don't plan to shoot" means "never point any weapon at something you don't plan to shoot". Well, I was taught "A circuit is always live". So never do anything that might create a path between that wire and ground, especially if that path involves your body. One day, about 30 years ago, I replaced a porch light. I had tripped the breaker on that circuit. I took the old fixture off, put the new fixture on, screwed in the bulb, and it lit. Oops. I guess the training of my youth paid off.
Higher voltages are more dangerous because they break down poor dielectrics faster. Remember, at all times, 15 mA across the heart is all it takes.