Does aldol condensation give different products in different conditions?
I believe your conception of the need for deprotonation under acidic conditions is misconceived.  Whenever you are under acidic conditions, your mechanisms must necessarily start with protonation.

The key concept here is the enol tautomerization.  The enol is more favored than for propanal as compared to ethanal because it results in more substituted alkene.  Thus there is likely a higher concentration of this nucleophile in the reaction mixture.  The mechanism of tautomerization is difficult to elucidate, but presumably starts with protonation of the carbonyl in acidic conditions.  Then, likely another carbonyl deprotonates the the recently protonated carbonyl at its alpha position, forming the enol.  This transformation has produced a nucleophile (enol -- 1-propene-1-ol ) and also a strong electrophile (a protonated carbonyl -- protonated ethanal).  The reaction occurs, leading to a protonated aldol product (proton on the carbonyl) which then is deprotonated by the next carbonyl and the process continues.

As far as the base catalyzed case, ethanal is more acidic because it does not have the extra methyl group donating electron density disfavoring formation of a negative charge.  Hence ethanal becomes the nucleophile and propanal becomes the electrophile.

In each case, these are only major products.  I don't expect the energy differences are large enough to give great selectivity.