I also have both.However, my rifled barrel is not a high end outfit, it's an 870 express barrel.
exactly what i had and with copper solids i regularly shot 2 1/2" groups outside to outside, so roughly 1 1/2" groups at 125 yards. this was with a scope mounted. i sold it and plan on replacing it with a dedicated slug gun but i'm not sure if that was s good idea or not...
the key to accuracy is finding the load that your setup likes. i got excellent groups with 2 3/4" copper solids, but 4-5" groups with the 3 inchers and was averaging 3" groups after a few different brands, but those copper solids were the ticket.
Better read this post again.
someone else should follow that advice as well.
to the OP...yes, the rifled barrel is almost always better and a scope will almost always increase your accuracy. therefore, if you opt to spend the extra money on a rifled barrel and mounting a scope you will almost always cut the size of groups in half of that of a smooth bore. the end result is being able to shoot with confidence at farther ranges and it's worth every penny.
once you have your gun dialed in, you leave your scope on the mount and its usually close to zero after switching from bird shot mode back to deer sniper mode

after that it's a box to rezero and make sure everything is good before hunting. then, you'll use one round for a deer. so,
maybe 2 boxes of ammo a year? but, you'll want to shoot it more than that anyways
as for holdover...
using hornady sst's and a 150 yard zero you'll be aiming bang on out to 175 and good to 225 yards if the accuracy is there. 1198 ft/lbs at 200 yards = dead deer.
Trajectory (inches)
Muzzle 50 100 150 200
-1.50 2.40 2.70 0.00 -6.70
Velocity (fps) / Energy (ft-lbs)
Muzzle............50..........100...........150...........200
2000/2664 1816/2196 1641/1793 1482/1463 1341/1198