Not much of one unless you measure directly in the flow of the fan. Since cases are pretty much a sieve---side panels aren't sealed, most cases have slot covers that have openings in them, etc., etc., it's pretty much as was stated above, in = out.
Yes... Not much of one, hence sneaky eyes. Although it is all part of a continuum.
I think this in = out idea is confusing people, of course after equilibrium in = out. A positive pressure case isn't always increasing in pressure. Imagine it as latency, the
in might have started first,
then the
out matches it's "speed". Having the same bandwidth =/ instantaneous.
I actually have 4 inlet fans in my case (no they are not running at the same time), if I turn them all on full speed it will still be in = out. But if someone tells me it's not positive pressure I'll think there's something wrong with them.
Edit: I think this was supposed to be more about the contiuum idea & less about bashing the in = out thing.