I would personally choose the 1815 up/down in rose gold. That is a truly beautiful timepiece. Don't be afraid to wear it with whatever the heck you feel like.
I'd say this is the most important part to address, because the two watches themselves are rather different in appearance and style although both are Langes. If the Saxonia Thin is looking like an alternative for you versus an 1815 Up/Down, I'd seriously consider the 1815 instead. Since both are serious considerations at their respective price points (and high ones at that), I wouldn't take anything but my number one choice.The 1815 is obviously much more expensive, but I really don't want any regret in the future simply because I want to save a bit of money now...
Though it's great that your wife would enjoy one of the watches, unless there's an additional sentimental component for this Lange between you and your wife, I wouldn't let that cloud your judgement too much if it's mostly going to be your watch. If it's getting the majority of wrist time on you, make sure it's dialed to your preference over any others.After hearing some of your stories and seeing the amazing photos, I leaned a bit more towards the 1815 UD. But then my wife said she really enjoyed the Saxonia Thin too....
I guess I will still think about it for some weeks...But having a watch that my wife can also `borrow' seems a strong argument in favor of the Saxonia Thin (I don't think the 1815 UD will look as good on her).