Pro racers can tell the diff,most mere mortals cannot. Shimano trickles their technology down,so today's 105 is better than 5 yr old Ultegra. If two bikes are close,say within $100,of each other in price,and one's 105 and the other Ultegra,then the Ultegra bike might be a better buy(if they didn't lower the other components). But I wouldn't spend $350 or more to get Ultegra unless there were alot of other upgrades as well.