if you have the money for the difference in price, ride hard and do a lot of miles, Ultegra is well worth it
the 6800 derailleurs and brakes last longer in our damp climate – you really see this in a busy workshop with the rear brake and front mech the first to go from constant back wheel spray
the 6800 crankset is stiffer in the big ring (5800 has nylon web insert, 6800 has carbon fibre insert), according to lab testing its the stiffest crank Shimano make if you value that sort of thing!
the 6800 shifters and caliper brakes definitely feel a touch smoother
I’ve spent a lot of time riding 5800 and 6800 and 5800-105 is simply fantastic, whereas 6800 is simply sublime 😉