Would you rather the citizen of Mars, Earth of the Belt? (The Expanse minus the aliens)
If you are already familiar with the Expanse series, you more or less know what the factions are about. The only difference here is no alien ever shows up. Things remain perfectly mundane and grounded.
A - Earth. Overcrowded and most are poor. You'd be able to get Universal Basic Income though. It's not much but will keep you alive. You would start in the lower middle class, but it is hard to keep that position. You could easily lose your income and end up at the bottom. It is incredibly difficult to move up unless you are exceptionally skilled or are willing to do something criminal.
B - Mars. Rewards hard work. Merit based society. If you contribute you are greatly rewarded but if you can't or won't, you are very likely to end up dead as the society would not have any resources to spare on you. Unlike Earth though, it is much easier to climb to the middle or even upper-middle class and maintain your position there. The poor are almost non-existent.
C - The Belt. Stronger sense of unity than the others. Everyone is poor and downtrodden however your side controls most of the resources in the system. You have the biggest hope for the future even though things kinda suck now but only if you get your act together. Getting free from the control of Mars and Earth won't be easy and this time there won't be any aliens coming to rile things up.