If it's important for your mobile workstation to be as portable and sexy as possible, then go ahead and get the Precision 5550. Otherwise, the older Precision 5540 will be cheaper and perform almost exactly the same.
https://www.notebookcheck.net/Dell-Precision-5550-is-the-full-on-refresh-of-the-Precision-5540-in-the-same-vein-as-the-XPS-15-9500-but-performance-hasn-t-really-changed.484380.0.html