I'm going to have to say the 38 looks better. Something about the way the smaller one works with the curvature of the wrist, and the proportion of visible band to watch face.
I've been leaning towards the 42 for a while (I have large wrists even for a guy), but if the battery life is the same on the 38 I'd consider getting one. I love the modern buckle leather band and I'm not so enthusiastic about the leather loop, so that's another plus for the 38.
Thank goodness I have two or so months to go until the watch comes out... Plenty of time to drive myself nuts thinking about which one to get.