My avatar has said it all.
Here is my reasoning, from objective perspective down to subjective and personal bias.
1. Assuming I liked both equally, I would keep Vianney Halter Goldpfeil because at least there are some other chronographs similar to the 1815 even for such a nice and special one, but there is absolutely no alternative to the Goldpfeil for that kind.
2. I happen to like Datograph more than 1815 Chronograph, and anyway I won't keep 1815.
3. As for Goldpfeil and Datograph, I like both very much to the point that I want to always keep them, and to the point that if I had to sell either one, I would most likely have to sell both.
But this is just me. You have a great collection of great watches Jay. However it seems neither Goldpfeil nor the Lange fits you and your collection very well, based on what you said. I feel it's just a matter of time that you will sell both eventually. In that regard it doesn't really matter which one to sell first. IIRC, you used to have Goldpfeil years ago. Did you buy it back recently? I do hope this time you will keep it.