I have the 500 GMT premium, initially intending to buy the 39mm version in steel. I have no idea why they are priced the same, as on the wrist they are distinctly different animals. The 42mm is a chunky beast, but as it's titanium I found it to be very wearable, the 39mm is really too small even for my 6.3/4" wrist (the wife has a 39mm pink MOP Ocean One). That being said, the build quality of both is pretty good at the price point.
If you are thinking of buying one, I would try them both first.