Given this compound in an acidic medium, what would get protonated first?
Would the double bond not get protonated preferentially, since oxygen is more electronegative than carbon and less willing to donate an electron pair for protonation?

I checked the $\mathrm{p}K_\mathrm{a}$ value for the conjugate acid of an  aldehyde, and it was around -10, which is pretty low; this suggests that the aldehyde would have a very low basicity.

Also ethene had a $\mathrm{p}K_\mathrm{a}$ value of around 43.

Going by this information does it not make sense for a double bond to get protonated first?

On the other hand, the oxonium ion is resonance stabilised, which would indicate that it is protonated in preference to the double bonds.

Which reasoning is correct? And more importantly, in these situations how are we supposed to predict which factor dominates?

  [1]: http://i.stack.imgur.com/zQaaD.png