i have the nug and the root and have ridden the harvest.
from reading your posts, I don't think you should get a nug. Take that money and fix your boots.
The twin shouldn't be that hard for you to turn, you definitely need to improve your technique. Going to a 150 rockered board will def make turning easier but it won't make you a better snowboarder at this point.
As Nivek said, getting a 57 to 59 medium stiffness board would help, but as long as your boots are killing you, you will not have fun on the hill.