Which Konosuke Petty, the 150mm or 180mm would better compliment my new 240mm Kon HD Gyutou?
I'm not a fan of longer petty's. I find them a bit unruly especially in a professional setting but this is really just personal preference. By the time I'm at 180+ I'm just grabbing a different blade so 150mm for me is an easy choice.
I like my 150 Kono quite a bit. 150 is a useful size for just about every paring, and all but the largest trimming and boning tasks. If you trim a lot of big pieces of meat, defatting brisket for instance, a 210 would be good; and a 180 would be an ideal size for a bar knife. I don't do enough precision work anymore to want anything shorter than 150.
FWIW, I don't notice significant flex with the 150; not even compared to my 6" Nogent slicer.