I have the Pebble Steel as well, and was pretty surprised reading the dimensions of the Apple watch when compared to the Steel. It's right in the middle of both models width wise, but still taller than both.
I've been leaning towards the 38mm, but every time I see these numbers, I feel like the 42mm would work just fine.
It really comes down to trying these things on, if only they had them for in-store demo before the pre-order date..
something I think about with the pebble is the prongs at the top where the band connects. I wonder if that is accounted for in the height.