I am not able to decide which among 1,3-butadiene and 2-butyne is the major product of this reaction. Will there be even a major product out of the two or will there be a mixture?
According to me, the product formed from the 1st mechanism should be the major product as it is more stable (two double bonds in a conjugate alkene are more stable than a triple bond). But I also think that product formed from mechanism II can be formed faster as the hydrogen being picked up in the first step of mechanism II is more acidic (due to the electron withdrawing effect of electronegative chlorine atom) as compared to the hydrogen being picked up in mechanism I.
Another factor, which seems to favor the formation of product by mechanism I, is the picking of hydrogen atom in the 2nd step which is easier from an sp3 carbon than from an sp2 carbon (in mechanism II).