Can't speak for the 39, but if you hold the original Pelagos next to a Black Bay, they are absolutely different creatures.
The Black Bay is clearly meant to be dressier, with a touch more elegance - the focus is the look.
The Pelagos is over-engineered, tough as nails, and Tudor's attempt to produce the most badass diver that modern technology can give - titanium construction, unbeatable legibility and the bezel, the crown and even the clasp are all clearly just highly technical and toolish. This is why it has remained as the last surviving luxury dive watch in my collection.
I personally don't care for the Pelagos becoming thinner, though I do feel that they could afford to make it in 39mm, and drop the date. Which they did, but the sunray finish detracts from the DNA of the Pelagos, which is why it doesn't sit well with me. I do feel it ends up making the Pelagos 39 take a step closer to the BB.. Not sure what Tudor is trying to achieve, but I suppose it will still sell reasonably well.
