I'm just saying its common sense because like I said, you have 6 less strings to worry about. Even if its not really a huge difference since they are in pairs, there are still 6 more strings. If there were no real difference (even a tiny one) people would say it doesnt matter. Learning would be more difficult (especially on your picking hand) when you start attempting to pick fast, or skip strings, ect. It all depends on the kind of music you play though. If all your playing is chords, then yes, a 21 string really makes no difference compared to a 6 string because your not really doing anything with individual strings.

Bottom line is, even if there is no big difference between them, noone can deny that there IS a difference, and LEARNING with a 6 stringer is always going to be easier just because of the fact that it has 6 individual strings instead of 12 (even if they are close together).