Tamislov....i weigh 80 kg and have the 21M. The fun range for me is sustatined wind of 9-13/14 knts. I like to ride fast and powered. I often see guys out on 14 (lighter guys) and 16's. They are just going while i am powered nicley.
I like the 21 better then the 18, but when i made the switch up i did the 21, 16, 12. I get great usable overlap with this spread. I would have been able to do the 14 / 21 but i was not convinced to get good overlap with skipping that many sizes, plus i felt naked without a 16M. The 14 would easily work at 13/14knts. That is a super kite and has good low end pull, imo.
I agree that the overlap between the 14 / 18 was not a lot. The 18 still wanted some wind on the bottom to really get working. I think you have to ask yourself if you like to hold a big kite in the 13 knt range, or whatever the bottom is for you with the 14M.