I've found that in the same year of manufacture, Ultegra shifts, feels and looks a little better than 105, and 105 shifts, feels and looks a little better than Tiagra. But I wouldn't hesitate to put a Tiagra group on anything. It comes to a value proposition- often the bike manufacturers put Ultegra or Dura-Ace on bikes with better wheels, frame, etc (which make more difference than the shifter group, IMO) and so Ultegra gets associated with a "better" bike. So, if you're comfortable spending the extra Benjamins, then throw on the Ultegra. If you want perfectly functional, then use Tiagra.