Like Darien, who just posted a thread about the price difference between these two models, I have been struggling for a while trying to decide which would be my first Lange.
The Lange 1 Moonphase was an early contender, but once I tried it on it felt small, so I felt inclined to go with the RL which felt perfect on my wrist.
However, I wanted it in "white" and the only alternative would be platinum...meaning quite some $$$ difference with the rose gold model which was an attractive reasonable alternative.
However, a year ago I saw the new 1815 come out with its very tempting 40mm size. And a very significant price difference with the RL in platinum. I will not argue obviously on the technical merits of the RL over the 1815, but I have to say that the small seconds and the blue hands grew on me and it all ended up pushing me just yesterday towards...THIS!!
The bottom line is that I am perfectly happy with the 1815 which will look terrific with a suit (I just did not felt like putting a jacket on on a Saturday ;-)) Entry level...but a beautiful one, and just what I wanted: an elegant & uncomplicated dress watch.