Worry more about the fit and the proper gearing to go with the smaller wheel size. As far as sora or tiagra they are both good compontents they are just not as "good" as the higher ultegra or Dur Ace. The biggers problem with the sora parts is they only go up to 8 speed, while tiagra you get 9 so you can interchange it with higher grade shimano parts. Note higher grade usually means lighter and made of aluminum and such. I have come to realize as long as it says Shimano, Campy, or SRAM it is quality and will work, if it doesn't then beware.