For me, the most condensed argument I can give is that anyone who says they know what will happen when we leave is massively deluded or lying. The idea that we will be a great trading power again by default is fanciful. And the other arguments for leaving just aren't that well justified, for me.
So I think it's better to stay in and have a voice, even though there needs to be a lot of change.
None of the main campaigns have represented the argument well whatsoever, though. I think it's a very complicated matter, that doesn't warrant simple narratives/headlines - so people should listen to the experts & academics.