In my experience this isn't that much better from a discomfort perspective, because (I would guess -- not an expert on physiology) the lower available oxygen in

In my experience this isn't that much better from a discomfort perspective, because (I would guess -- not an expert on physiology) the lower available oxygen in the lungs also leads to a more rapid increase in blood CO2. It might be a bit better depending on your particular body though, so yes it's worth trying both out. Actually I just tried it and yeah for me at least the full-lungs version is obviously longer but takes less discomfort for my SpO2 to start to drop (like well before contractions start), whereas on exhale I get more discomfort (around 15s before contractions) before a similar drop.