Ken - considering your current collection, I recommend going with the Datograph next, the beauty of the Lange chrono movements is unsurpassed as you know well. My personal preference would be the Datograph in red gold since you have an 1815 Chrono. I have an 1815 Chrono too, and chose a Dato in RG rather than PT as it makes a nice contrast with the 1815 Chrono. There is something to be said for diverstiy, and my own collection features one Lange in each metal. I like the variety!