Ballistics by the Inch shows the 16" barrel with a higher muzzle velocity than the 20" length though not by much.
I have the Big Boy Classic in 357 Mag with the brass receiver and everything. Mine has the 20 inch barrel. My Henry is one of my very favorite firearms. The 20 inch is my recommendation. Plus, you should get better velocity and sighting with the additional barrel length. This is one high quality firearm. I think a matching Henry 30-30 may be my next purchase.I can't make up my mind on 16.5 carbine, or 20". Both Octagon barrel. I have both in other calibers but having a tough time on 357. Either will be a case hardened. Thanks!