Especially for certain components there is little to no difference. Front derailleurs get slightly lighter but don't work differently, while rears have more of a weight difference (for whatever it matters) and can be snappier, especially going from a low-end to mid or upper. On the other hand, from what I have read the early 6700 shifters tended to fray and cut shifting cables due to an engineering issue that was rectified for 5700, making the lower-end option preferable of those two.

Over in Campy-land, going to Chorus shifters instead of Athena allows more gears to be shifted at once in each direction, and the crank gets an appreciable upgrade as well.

Now, how much all this means to a recreation rider is an entirely different question.