The thing with 105 vs Tiagra right now is that 105 is 11-speed and compatible with other 11-speed parts from 105, Ultegra, and Dura-Ace. Current Tiagra is 10-speed, so it's not compatible, and is also a different 10-speed from older 10-speed, so even older 10-speed parts are not compatible. That sort of situation makes me uneasy.