I'm a woman, and I have a 38mm SS lt pink medium modern buckle. I also have been eyeing the ML as well as the Link Bracelet. (I've worn a stretch link bracelet on a Seiko Micky watch for over 22 years prior to getting the Apple Watch.)
I have tiny wrists, but am also chubby. Because my wrist bones are so small, I opted for the 38 and this is the right choice for me. I'd love to have the 42, but think it would overpower my wrist. Although my bone is small, because of the extra flesh I have on my wrist, I have the med buckle on the loosest hole, and that just barely works for me. One hole tighter and it squishes the skin, even though it has plenty of give on the wrist bone itself.
Looking at your wrist, and knowing what I've learned about fit, I'd recommend keeping the 42 order with the ML and trying it before you make a final decision. From your picture, I agree that the ML just barely fits, with not much room to spare. I don't think I'd want the magnet/clasp so visible in the front next to the watch face if it were mine. The bigger watch face and slightly wider/longer band might give you the perfect fit you're looking for. It's at least worth trying.
As several have mentioned, if you're not able to return the earlier watch (which they still may let you do if it's in great condition when you receive the later watch and you talk to the right person about it), you can probably pretty easily sell it on ebay, Craigslist, or through word of mouth.