This is an excellent question. Juvederm Ultra XC and Juvederm Volift XC (now marketed as Juvederm Vollure in the United States) are both excellent options for lip augmentation. They are designed and formulated slightly differently, and as a result, are used for different indication. In my experience, Juvederm Ultra is a better for adding more dramatic structure and volume to the lips, while Volift/Vollure is more for subtle enhancement. Volift/Vollure has only been available in the US for a few weeks now, so I don't have personal experience as to its longevity in the lips - that said, based on the scientific studies conducted before its American launch, I believe it will likely last longer than Juvederm Ultra.
Ultimately, product selection depends most on your facial structure/anatomy, the existing shape and volume of your lips, and your aesthetic goals. I would recommend seeing a cosmetic dermatologist or plastic surgeon to discuss the best product for you. Best of luck!