[amluto]

A quick search for higher heating values suggests that acetylene and hydrogen gasses have fairly similar HHV per mole of oxidizer. (H2 needs 1/2 equivalent or O2; acetylene needs 5/2 equivalents, so H2 wins by a bit.)

But H2 takes up most of the space in the balloon, and acetylene is nice and compact, so considerably more total energy should be available with acetylene!

I don’t know whether oxyacetylene will detonate nicely, though, or whether a balloon-sized oxyacetylene mix will merely combust subsonically.