Maybe +1 AC when used one-handed, and +1 to hit when used two-handed would cover it? I'd like something more interesting than flat bonuses, myself, but that'd model what you describe in a preponderance of the evidence kind of way, and it's a clear choice, that synergizes with the option of a shield one-handed, and the larger damage die 2-handed. Defense style already gives a +1 AC, so there's a stacking danger, there, and +1 AC is a big deal since AC currently scales only with DEX and gear. It might be desireable to phrase it in a way that doesn't stack with Defense style (then again, the only ones who get two styles that I recall are Champions, and it's not like they're in danger of dominating the game as it is).