For years Ultegra mechanical was the sweet spot for quality vs. cost on the Shimano price curve - 95% of the functionality of Dura Ace for like 60% of the cost - better for nearly everyone
I’d propose that your real decision should be between Ultegra mechanical and Ultegra Di2. Look at the cost difference between those two groups and decide what you would want to purchase with that money and how that compares to the difference between mechanical and Di2
Given your two choices Ultegra Di2 is a no-brainer.
Given a limited budget and a potential list of other upgrades (higher quality frame, better wheels, power meter for the bike, etc.) I might recommend Ultegra mechanical so you can get something else nice on the bike