A wider tail makes it more difficult to turn, but with your weight that is not the problem. And if you want to surf it as well: def. a retro fish!
People saying that over 19 is no fun never used a good retro fish in low, mushy waves, or are not that heavy.
Most kitesurfboards are designed for an 80 kg person, remember that!
Can only agree - weight makes even big boards seem small, so you can go really wide
Kiteguy, you write your wind speeds are 10 to 20 knots ?
At 10 knots you can not ride on anything...
Around 14 knots (in warm weather I assume), you can slowly start riding waves (not good though, barely possible) with a really big surfboard.
But at 20 knots, you'll be more than fine with a medium surfboard - as this is A LOT OF WIND, compared to 14 (more than twice as much power).
So IMO quite important if you most often ride in the lower end - as then no board can be too big for your weight, or you ride quite often at 18-20 knots where you can use smaller kites and boards with better and more natural "agility"
I assume you will choose something a bit "in between" though - quite big and wide, but not extreme, and not too flat either.
Would just point out that the useable range from 14 to 20 knots is really different at the ends, and can not be compared whatsoever
(and less than 14 knots is useless...)