.. for me, at least, as both watches are one step away from perfection in my book. As someone who finds an IWC Big Pilot to be the perfect everyday watch, the 39mm Data just feels small. I have tried to fight this feeling for 6 years, because I'd love to go for a Dato, but it is what it is. The DS, on the other hand, feels perfect on my wrist - but I always felt something was missing. I think I've finally worked it out - I'm not a fan of the bridge over the balance, as you can no longer see the "heart" of the watch. And it is also, frankly, out of my price range.
So - that leaves me desparely wanting a 42mm Datograph!! I think it would actually improve the balance of the watch, which IMO is a tad thick at its current 39mm. I am sure I am in the minority here, but you can always hope...