Between ethene and propene which is more acidic and why?
The difference between there acidity arises only due to the difference in inductive effect[or the electron donating capacity of the alkyl group] of the alkyl group attached to the $\ce{C=C}$ in both the cases.

By this site there exists a difference of total one unit

For more info on Inductive Effect

Relative acidity of a compound is checked by removing the acidic hydrogen and then comparing the stability of the anions.The more stable the anion is the faster it is formed and hence the acid is stronger.

The anions formed are $\ce{H3C-CH=CH-}$ and  $\ce{H2C=CH-}$ . In the former one the anion or the negative charge on the anion is destabilized by the electron donating ability or the inductive effect of $\ce{CH3-}$, hence it is less stable and less rapidly formed and hence less acidic. 


  Why $\ce{H3C-}$ has electron donating effect?


In an alkyl group, for example, in methyl group carbon is linked to three hydrogen atoms. Carbon is more electronegative than hydrogen. So carbon pulls bonded electrons nearer to carbon and carbon acquires a slight extra negative charge (delta-) and correspondingly hydrogen lose hold over bonded electrons and acquires a slight positive charge (indicated by delta +). Thus carbon get a little more electronic charge which the carbon will push towards the group to which it is linked.When this electron rich carbon centre (methyl group) attached with other compounds it behaves as electron donor. 
And thus methyl group acts as electron donating group. 
Be in mind, the electronegativity difference between carbon and hydrogen is very less.  

$\ce{H3C-CH=CH-}$ is less stable due to presence of above factor.

You can also use conjugate base method or charge holding capacity method to compare the acidity of compound to give the same result as above.