Congratulations on making a very difficult decision. Practically twins on paper. Starting in 2013 I starting looking very closely at the Lange 1815 Chrono. Benefited from several opportunities to evaluate this reference and spent many years trying to love it. During a 12 year hunt for a hand wound chronograph I continued to return to Lange 1815 vs Patek 5170. I held the Lange in my hands, louped it and wore it. Every time I held the Lange I came to the same conclusion. Even though years had passed I was reminded that nothing had change in my opinion: the backside view (glorious) was not enough to make it win over the Patek 5170 attributes. In that same long time period I only had one occasion to hold a Patek 5170 and that was in the early days of my hunt. The Patek I held was the 5170J - my least favorite variation. The J was too staid, too cluttered with Doctor's watch cues, the Roman Numerals inner location wasn't to my liking, I don't particularly love yellow gold. Despite all that I was still impressed with it. That was in 2014. All the other Patek boutique visits were unsuccessful in the 5170 hunt. My success rate at Lange boutiques was higher despite visiting Patek more. Nearly every visit to Lange produced a tray full of choices. What a difference from Patek. Not in a good way. I tried to NOT let that constant availability (and what that said about Lange desirability) cloud my decision. The watch comparison had to be based solely on all the merits of the two watches.
I've already been beat up for allowing her to stack it with evil looking bracelets that might scratch the Patek LOL!