Bigger screen, bigger battery. They cancel out.
No, they don't. It's bigger enough that it will add time to overall battery life. The battery grows in three dimensions while the screen only in two, and all evidence suggests the screen is OLED (direct or just lit, either way, that's defining the power requirements.)
Anyway, more importantly, Apple says the 42mm will experience longer battery life. End of story.
http://www.apple.com/watch/battery.html
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The width is the biggest difference. I do not mind the height of the 42mm, but I think the width is a little ridiculous.
The width is only 36mm on the 42mm model. That's smaller than an average classic styled men's wristwatch which are typically 38-40mm. The thing is, the sizes quoted by the watch industry usually describe the width while Apple is describing the height.
Also, it's worth pointing out that the 38mm version is actually 38.6mm. There is really only 3.4mm difference in height.
I won't say that this tiny difference will never matter, but, depending on the type of band, it won't even be perceivable. It's a bigger deal if you're sporting average or smaller wrists (170mm and lower) and want the classic buckle or Milanese loop as they both add a significant amount to the height of the watch with their extended lugs. But, if you're going for the leather loop, sport, or link bracelet, the 42 is not going to look much different than the 38 on an average wrist.
Something to keep in mind is that watch styles for rectangular watches, in terms of fit, are different from round ones. Since the watch face and band are similar in width for some of the band styles, it will be irrelevant as long as the watch doesn't extend beyond the surface of your wrist. The biggest factor will be the width and 36mm is small by today's standards for all but the very smallest wrists.
Yesterday Apple posted some video guides for the Apple Watch. Two have a male model wrist and two have a female model. Watch them. If you think their fit is appropriate, realize that the male model has about 165mm wrists, maybe slightly smaller and is wearing the 42mm in both videos. And yes, I took the time to capture a frame and scale it appropriate to figure this out. It's hard to find good examples of this watch or anything similar on a wrist with a good view.
Anyway, if that doesn't look too big to someone, then it should help answer their questions about size. I didn't take the time to do the same size comparison for the woman's wrist, but, am guessing it's average for a woman and sporting the 38mm model. I assume Apple made these model choices deliberately to demonstrate the appearance they expect for sizes they expect people to be choosing.
Watch! I know a lot of the forum is still deciding between the 38 MM or 42 MM face. I was wondering if 4 MM will really make that much of a difference. While cutting out templates I realized that 4 MM (the difference between the 2 watches) is a very small amount. in the end do you think is really matters, if I preorder incorrectly. 
