The RMS (root-mean square) value of an AC voltage, which is what is represented as "110 V" or "120 V" or "240 V" is lower than the electricity's peak voltage. Alternating current has a sinusoidal voltage, that's how it alternates. So yes, it's more than it appears, but not by a terrific amount. 120 V RMS turns out to be about 170 V peak-to-ground.
I remember hearing once that it is current, not voltage, that is dangerous to the human body. This page describes it well. According to them, if more than 100 mA makes it through your body, AC or DC, you're probably dead.
One of the reasons that AC might be considered more dangerous is that it arguably has more ways of getting into your body. Since the voltage alternates, it can cause current to enter and exit your body even without a closed loop, since your body (and what ground it's attached to) has capacitance. DC cannot do that. Also, AC is quite easily stepped up to higher voltages using transformers, while with DC that requires some relatively elaborate electronics. Finally, while your skin has a fairly high resistance to protect you, and the air is also a terrific insulator as long as you're not touching any wires, sometimes the inductance of AC transformers can cause high-voltage sparks that break down the air and I imagine can get through your skin a bit as well.
Also, like you mentioned, the heart is controlled by electric pulses and repeated pulses of electricity can throw this off quite a bit and cause a heart attack. However, I don't think that this is unique to alternating current. I read once about an unfortunate young man that was learning about electricity and wanted to measure the resistance of his own body. He took a multimeter and set a lead to each thumb. By accident or by stupidity, he punctured both thumbs with the leads, and the small (I imagine it to be 9 V) battery in the multimeter caused a current in his bloodstream, and he died on the spot. So maybe ignorance is more dangerous than either AC or DC.