Well, both are true. More rifling engagement from 1" on equals more stabilization - up to a point, then gets severely diminishing returns. A shorter barrel coming down from 35" or 40", let's say, provides more stiffness - up to a point, then gets severely diminishing returns. That's why neither extreme is the best; the optimal is somewhere in between. Evidently, with a *sporter weight* barrel, the optimal point comes in at right about 21.75" for best accuracy. Intensive research has confirmed that the goldilocks point is somewhere right about in there. With heavier barrels, it's a bit longer, at 24 inches or more, depending upon HOW heavy the barrel is. It's a complex calculus of interconnected and competing factors affecting accuracy. But you CAN generalize:
For barrels between 1" and 15", a longer barrel will tend to be a bit more accurate, ceterus paribus.
For barrels between 20" and 40", a shorter barrel will tend to be a bit more accurate, ceteris paribus.
For between 15" and 20", it's about a wash - I don't know if you can even say.