I don't think there is any just one right answer to this, think it depends more on your particular needs, preferences, average climate conditions, etc.
Personally, i do a lot of trail hiking with minimal bushwhacking, and since i live in and primarily hike in central to western part of VA where there aren't a lot of extremes (except for maybe heat), i like an oversized windshirt that gives me more flexibility with how much i can layer under it, more freedom of movement, and more innate aeration if i want or need it (by reducing layers, leaving the bottom more loose, etc).
If i did a lot of bushwhacking, or mountaineering, i would probably want a more slim fitting windshirt to minimize damage to same, for one example.
Whatever works for you. Some (or a lot) of this stuff is relative and variable.
Edit: just adding that one of the more specific reasons of why i have an oversized windshirt is so that i can put it over my puffy insulation jacket to preserve same from damage. Not that i hike in it, but since the shells are a bit fragile, and i'm cheap, i want to preserve it for a long time and since i actually do more "bushwhacking" usually when camped, than when actually hiking, well makes sense for me.