Hi Laurie

Hydrogen might be trickier to store for the long transit to the target palnet whereas keeping He under pressure in space is a well developed art.

Also hydrogen and carbon dioxide is a flammable mixture - their burning is known as the Bosch reaction, which is exothermic and produces water and carbon. Hydrogen is a potential fuel for jets and IC engines on Mars, though sooty build up in the exhaust system would need a periodic purge of straight O2 to burn it up.

Nick Hoffman pointed out this concern when I discussed hydrogen balloons on Venus.

The gain in lift for Hydrogen (m = 2.016) and helium (m = 4.0) in a CO2/N2 mix (m = 43.5) is an extra 2 kg per kilomole of gas, which seems minimal. With large enough balloons a breathable N2/O2 mix (m = 28.9) is a lifting gas. You could, in principle, build floating colonies on a Mars with a denser atmosphere. Such are feasible - if not desirable - on Venus.

Adam