Moth, please use cm (i.e. metric in general), especially if you’re used to it. This is a worldwide forum and, by far, most people worldwide use the metric system. So if you convert your inseam to inches, most people have to convert back to cm which may (again :)) introduce errors. The one notable exception is wheel size: everyone here is used to quote those in inches.
Back to your question: definitely get a 24" for off-road. Not only faster to ride but also easier, the wheel rolls better over irregular surface. And your inseam is big enough.
Moth, didn’t you mention in another thread that you were average height for your age? The measurement you gave sounds pretty small. How tall are you (use whatever increment works for you)?
Since you are still small and this is prob your first good uni I’d go w/ a 19" trials, which you could also use for Muni, flat, and street (tricks off/onto obstacles, like stairs). I’ve only ever seen one kid ride a 24" instead of a trials for Muni, and I think he was 10.
I’ve never seen a small rider use a 24 who didn’t have at least a couple of inches they could lower the post if they wanted to. Besides according to UDC you’d need an inseam of 29" on a Nimbus Muni w/o cutting the frame.