Can't tell you about sizing comparison because I haven't tried the full-zip, but I know it runs smaller than older Ibex designs (like the Carrie hoodie, which is too big for me even in a S). I layer with only one thin athletic layer, there wouldn't be that much room under it, but your experience will vary according to your size of course.
I wear it all the time, way too much in fact. I got mine in black and it's really versatile. Some days I wished it didn't have a hood, but I really like the thumbholes.