I recommend going with the 42 mm unless you have an extremely small frame and extra small wrists. The 42 will fit fine even if you have small wrists - just use the small/medium band. This isn't a watch that is meant to be dainty. It's a touchscreen smartwatch that displays text and you need the screen real estate. I tried on my brother's 42 mm Apple Watch a couple days ago and it fit great. I have other women's watches that are about the same size and I love the fit of them as well.
I originally thought the 42 was the better choice, so I ordered it. But after trying both on twice, with different bands went with the 38.
I am extremely happy that I downsized. My wrists are extra small and the 42 was just a tad too big. I had to pull the sports band really tight to get a good fit. Even then the band didn't line up as comfortably as the 38 does.
It also helped that the 42 was back ordered til June and I was able to get the 38 in record time.