As someone who quickly sold off his Kickstarter Pebble b/c it was too damn big and clunky on my wrist, yes, if I get an Apple Watch it will be 100% the 38mm one. If it only came in 42mm I would even be considering it.
I used the link on the previous page to the templates of the two sizes and compared it to my Pebble and it looks like the 42mm is similar in size to the Pebble. I still have my Pebble and have no issue with its size so I'm inclined to go with the 42mm Watch.
so i printed the sizing comparison PDF and compared it to my pebble - i'm really suprised! - Looking at the 42mm model - its about the same size. It's about the same thickness (the sensors stick out a tiny bit further than the pebble) its not as tall but a little bit thicker. The pictures make it look huge - but its not at all. I'm totally sold on the 42mm version.