I saw this advice a long time ago, it goes roughly, is the story you’re telling the most interesting part of the story, if not why?
You gotta make your reader care about your story first before worrying too much about the narrative order.
If you're going to start with the prequel spin off, you have to make sure that the reader doesn't feel like they are reading a side show, with more interesting parts happening in the background.
If you start with the main story, you have to ask yourself, how relevant the events are to your story. If the beginning of the main story doesn’t make sense if you haven’t read the spin off, then it probably means you should start with the prequel.
I hope this helps.