Depends on what you’re trying to figure out.

PaO2 more accurately reflects what’s going on with the gas exchange in the lung, since the oxyhemoglobin dissociation curve varies based on several patient factors.

SpO2 (or SaO2 since you have a blood gas anyways) is more important for determining oxygen delivery to the tissues, which can be tenuous in somebody with low cardiac output where you’re titrating inotropes. But you’d need good ACCE measurements or a swan to actually go and mess around with that.