It's mostly a matter of preference on air flow direction. There are pluses and minuses to each approach. There's also a more subtle effect, which is the aero- and fluid-dynamic view. This can be a big factor in audible noise and conversion efficiency of motor work to air flow.
For instance, if you look at the inlet versus outlet conditions of any fan (a motor, a mounting/spacing function or venturi system, and an aerodynamic rotating blade), it's not symmetric unless there's quite a long "pipe" section at each end to achieve laminar flow. I'm pretty sure most fans are specified and rated according to certain standardized physical arrangements that are hardly ever realized in practice - at least not in a compact form inside a piece of equipment. In other words, the vacuum side acts quite differently from the pressure side, depending on the immediate physical environments.
You can observe this experimentally if desired, with a fan running out in the open, and some simple tests. The main point is that the vacuum side is weakest, limited to one atmosphere of working pressure no matter what - it can be a fan or a pump/compressor or a (normally-aspirated) internal combustion engine.
I'll say some more next time.
Ed
