IMO its more than just weight, there's a massive difference between Sora and 105 in terms of operation (I believe Tiagra is similar to 105). Sora uses a sideways motion of the brake lever to change gear one way and a thumb trigger to change the other (I don't like the trigger its too small and I and I believe most folk can only change from the hoods), 105 has the same brake lever motion but instead has a lever behind to change the other way which can be reached both when you are on the hoods and drops. 105 seems more comfortable than sora but that might be because my 105 is on a better frame.