Overclocking versus Overvolting in Raspberry Pi. Which one is more dangerous ? Why ?

I would say OV is more deadlier than OC

Most transistor in any IC are not run at 100% of their effective speed. So OC ing will not destroy my chip.

Whereas their voltage input tolerance is not upto the mark and their range of operation is limited. So OV can destroy the IC and doesn't allow the chip to work at all

 

Overvolting = more heat, bandwidth effects. Causes metal migration phenomena and electrochemical corrosion over time. It also causes ionic displacement and exacerbates leakage currents (= more heat). On a whole overvolting will eventually destroy your chip. Overvolting also has an effect on internal resistances and capacitances of the material and device, and as such may change the V-I characteristics or region of operation away from the intended Q-point/region.

Overclocking is generally a lot safer mainly because you are dealing mainly with heat effects only and not so much with corrosive phenomena. Thus, overclocking may mostly only cause chip problems at operation time, overvolting could even cause explosions (and irreversible damage)

 

^The thread gives an overly simplistic definition of both overclocking and overvolting. The effects seen from overclocking are simply a result of the limitation on gain bandwidth product of any transistor based circuit on high frequency. By manipulating the signal such that a tolerance value is reached quicker (by raising the design limit), overvolting works to achieve something similar.

The issue of why overvolting is bad is not explained very well in that thread. It is bad basically because it decreases the thermal, chemical and electrical efficiency of the carrier conduction process. Each time energy is not used efficiently, the remainder is converted to heat. That's why more volts = more heat. We then have electromigration and other chemical effects to consider.