if you aren't getting good patterns with the Mossberg try tinkering with different chokes and loads, the length of the barrel shouldn't make too much of a difference. As for which 870 to get i really like the shorter barrel on a turkey gun, especially because i like to run and gun so my shotgun spends a lot of time on my shoulder as i cover a lot of ground. You could always sell the mossberg and buy another barrel for the 870 so you will have a turkey barrel and a wingshooting barrel