I think the big distinction between Loners and Bandits is that Loners, most of the time, will leave you alone and have a loose fraternity with each other. They don't work together, and they won't go out of their way to save you unless you have a personal connection, but they do have a loose code of conduct and mutual respect. They're the survivors, the guides, the people who map the zone and understand and respect it's secrets. Their goal is to exploit the zone for whatever they can find that is of value, and survive.

The Bandits, on the other hand, are mostly petty criminals with very little experience in the Zone. Whereas the Loners want to find riches in the zone, the Bandits explicitly want to take from other people