From what I heard, slave system in Ideology isn't really worth it since they rebel too easily and its hard to keep them suppressed without mods.

You can get slaves to do almost anything a normal colonist can do and so they can be easily tasked with stuff like hauling, at the risk of them rebelling if not kept in line. Prisoners have a more restricted range of tasks and movement, but aren't any more likely to stage a jailbreak than they'd be normally. Good for crafting stone blocks and other stuff, so long as you can keep the materials they need for it nearby and don't let them craft weapons. For both I tend to have a dedicated warden or two in big colonies to keep them in check.
I think it adds a little bit of flavor to a run personally, thought you could certainly choose one over the other and get more or less the same benefits in the form of free/low cost labor

I just noticed vanilla game doesn't allow prisoners to "social“ (like chat, or even romance) with other prisoners from different factions. It seems to be boring when you own a big mixed prison. However, I remember my prisoners could social in 1.2, with Prison Labor mod. Maybe it's a feature of this mod but I'm not sure.