Ooo! One that I can kind of answer! The first thing you need to understand about the killing process in the holocaust is that it wasn't as regimented or organized as you think. Not to digress too much, but in the field of holocaust studies, a lot of us aren't even calling the holocaust one event, but are rather referring to it as serial genocides (yes, plural).

Experimentation with killing and labor methods was the norm until about 1943 or so. Auschwitz was also about 50 total camps and had most major groups represented, including Soviet and British POWs. Soviet POWs were the first to be gassed with Zyklon B, and the British POWs even fed the camp's underground economy, as Primo Levi mentions in Survival in Auschwitz.

Auschwitz also was in a strange category of camp. It was both a labor and an extermination camp. If you weren't sent to the chambers immediately, you could expect to survive for a short amount of time before being deemed unfit to work. This also depended on your job. If you had a technical job like Levi, a chemist, you had a longer life expectancy. If you had an outside or physical job, you may go to the chambers in a week, even.

What I'm getting at here is that this question is impossible to answer definitively, because even camps themselves were inconsistent.

Saint Maximillian Kolbe, as I recall, volunteered to go into a starvation hut in the place of a man randomly selected. This man also cried that he had a wife and/or child. When Kolbe volunteered to replace this man, it would make no difference to the Germans, frankly. This was purely a numbers game. One escapes, ten die. Who made up the ten didn't entirely matter. Could be random, could be others in your building.

So the short answer is, we don't know. But it's likely that in situations like this, it frankly did not matter who went to be killed, so long as it was someone