I guess that if I could use a 10mtr instead of a 6m I'd rather be on the 10m...
Depends, My current ones overlap from say 20 to 30 knots at the extreme. In 20 knots for sure most times I'd rather the 10m, In 28, not so much. Even in a steady 22 ish, If its waves, Im going for the 6m every time.
20 knots is kinda magic wind where anything works. Same rider can hold down a 12 comfy enough and boost or zip around in waves on a surfboard with half the kite.
There are also drastically different 6m kites. My first was gutless and took a solid 25 to have any fun. Had a 10m and 14m of the same kite so it all worked out. Now I get the same range from just a 6m and 10m, they just have loads more low end. My current one starts working well in as little as 20 knots. I personally have toped out on it somewhere around 40. Those are also drastically different days. Accordingly my experiences with 6m kites is varied, but pretty much all good. The percentage of 6m days that I actually remember is much higher than 10m days!